I can confirm this bug.  I have the same results with identical setup.

As a workaround, have you looked at
http://sourceforge.net/mailarchive/message.php?msg_id=25627567
?

Hey Oliver, yes, I've seen and tried that. That managed remove the bulk of it. I don't know how that binary junk got in there, but thankfully it's gone.

I seem to have identified another problem massively contributing to bloated save files. A friend of mine sent me a project file that I already had, but he had made some minor changes, saved, and then emailed the project file to me. I noticed his was only 1 MB, whereas mine was 3.4 MB. When I opened his and resaved it, it then bloated to 7.4 MB. It stayed at 7.4 MB if I kept resaving.

I took a look at a diff between his project and mine, since we hadn't really changed much, and I saw the following...

The end of his project file contained:
  (...)
  </listview>
  <codegeneration>
   <codegenerator language="C++"/>
  </codegeneration>
 </XMI.extensions>
</XMI>

In mine on the other hand, sandwiched between the codegeneration tags is 47007 lines of code generator elements...

(...)
  </listview>
  <codegeneration>
   <codegenerator language="C++">
(...)
    <classifiercodedocument package="Logical View" fileExt=".h" parent_class="jodyPkDFUDsT" id="cppheaderjodyPkDFUDsT" fileName="const_char_" writeOutCode="true">
     <textblocks/>
     <header>
      <codecomment tag=""/>
     </header>
     <classfields/>
    </classifiercodedocument>
   </codegenerator>
  </codegeneration>
 </XMI.extensions>
</XMI>

Either he and I have different Umbrello configuration settings or Umbrello has a bug where it's inserting the code generator output. If the former, where do I disable this?

I guess that the different sizes of the xmi-files is because of the different set checkbox "Use new C++/Java/Ruby generators" in the general settings dialog.
If the checkbox is selected, code documents are written to the xmi-file.

Hey Andi. I can confirm that you seem to have found the culprit. A couple observations:

(1) The new code generator does not appear to scale well for a large project, like the one I showed you.

(2) If it needs to generate what it did when the feature is enabled, then perhaps we should consider automatic transparent gzip'ing of project files.

Something else I noticed with that project file when trying to export diagram images from the console was the file appears to still have a great deal of garbage in it:

$ umbrello --export svg Engine.xmi
umbrello(644)/kdeui (kdelibs): Attempt to use QAction "edit_undo" with KXMLGUIFactory! 
umbrello(644)/kdeui (kdelibs): Attempt to use QAction "edit_redo" with KXMLGUIFactory! 
umbrello(644) KXMLGUI::ActionList::plug: Index  18  is not within range (0 -  11 
umbrello(644): Guess is Uml::ModelType::N_MODELTYPES - package not set correctly for  ""  / base type  116 
...
< repeats a gazillion times >
...
umbrello(644): Guess is Uml::ModelType::N_MODELTYPES - package not set correctly for  ""  / base type  116 
umbrello(644): loadFromXMI(UMLRole): id  "eGBqqwbL28pg"  is already in use!!! Please fix your XMI file. 
...
< repeats a gazillion times with different id >
...
umbrello(644): loadFromXMI(UMLRole): id  "8rcMzyH06JU6"  is already in use!!! Please fix your XMI file. 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
...
< repeats a gazillion times >
...
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644)/kdecore (KConfigSkeleton) KCoreConfigSkeleton::writeConfig:
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLListView::findView: returning 0 at UMLListView::findView 
umbrello(644) UMLCanvasObject::removeAssociationEnd: can not find given assoc in list 
< repeats a gazillion times >
umbrello(644) UMLCanvasObject::removeAssociationEnd: can not find given assoc in list

I need help - I made project in Umbrello , few diagrams, and some classes ... XMI file that has been generated by Umbrello has now 1.1 GB! Umbrello crash when is trying to save it, and opening of this file last forever. Is there any way to compress (compact, clean) XMI file ? 

My Umbrello version is 2.11.5.

Hey Sebastian. Sorry to hear that. Oliver has a link in comment #2 to a script (assuming that was it) that removed the junk out of the XMI file. I see the link is dead, but I have the script saved still and I've uploaded here for you as an attachment. Hopefully this works for you.

Created attachment 87949 [details]
Hack bash script to remove junk from XMI file.

Oh man!! THX! :) My XMI is alive again.

Created attachment 87956 [details]
signature.asc

On Fri, 2014-07-25 at 21:36 +0000, Sebastian wrote:
> Oh man!! THX! :) My XMI is alive again.

Right on Sebastian! Glad to hear.

Thank you for the bug report.

As this report hasn't seen any changes in 5 years or more, we ask if you can please confirm that the issue still persists.

If this bug is no longer persisting or relevant please change the status to resolved.

(In reply to Justin Zobel from comment #12)
> Thank you for the bug report.
> 
> As this report hasn't seen any changes in 5 years or more, we ask if you can
> please confirm that the issue still persists.
> 
> If this bug is no longer persisting or relevant please change the status to
> resolved.

Hey Justin. I actually haven't used Umbrello in years, so unfortunately I'm not in a position to verify.