SUMMARY
from kde forum. we moved.
https://forum.kde.org/viewtopic.php?f=139&t=156138

hi. we add 1 img to .kra.
it's 200kb. not big. size Width 520 x Height 831. .kra canbas 1024 x 1024.

STEPS TO REPRODUCE1
1. create .kra with template texture 1024 8bit is 428.6KiB.
2. paste img. 428.6Kib
3. .kra save.
OBSERVED RESULT
.kra 428,6KiB to 1.0MiB. so .kra size is bigger file size.

OBSERVED RESULT2
1. make .kra 2.1MB. have 2Layer and layer style pattern overlay<not important.
2. add image is 861.3KiB.
3. .kra save.
OBSERVED RESULT
krita change .kra size 5.8MB.

why bigger file size? strange. 
why problem? becouse it is krita size bigger = use an texture image difficult.
if you are have result file. different filesize.
A. use paste image .kra to export file size is many big. A is 3.6MiB.
B. nothing have paste image .kra = export file size is small. B is 2.6MiB.
200KiB image but result file size different 1MiB.
paste image is icon. no tiling carpet. no using layer overlay. not change.

EXPECTED RESULT
.kra file size too many small.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma:5.12.7
KDE Frameworks Version: 5.47.0
Qt Version: 5.9.5