Brush size increment/decrement shortcuts affect all brush sizes in the program other than the liquify tool 'brush' size. Don't know if intentional or an oversight. 4.4.4 git cc5d52c
Neither: liquify isn't a regular brush engine, but a transform tool engine, so it's not in the same category. It would be useful to make it work like brush engines, but that's a feature request, not a bug.
(In reply to Halla Rempt from comment #1) Noted, I'll start up a feature request