3.4.0 introduces a lot of rtl support which means some code duplication.
It seems like it gonna be reverted though, and ltr will be defaulted, so you get a 3% decrease in bundle size over 3.3.0.
That PR was actually to fix stuff like:
margin-top: 5px;
margin-bottom: 5px;
margin-left: 5px;
margin-right: 5px;
to be margin: 5px
Which I believe is the 23kb difference you see in pure ltr.
However, because of css specificity rules, we had to move a bunch of code to be under [dir=...].
Talking about the real numbers (pure framework, everything at default):
ltr: 375kb, rtl: 378kb, default(ltr+rtl): 481kb
So correct, there is a 28% increase to the core. (a lot, and there is a bit more coming)
I am guessing your bundle size is so large because perhaps you are using a lot of colors. Every color is an additional ~50kb.
When I am done adding more and more to the bundle, the next step will be to move this to app-scripts, which reduces the bundle size by 3%.
I was also thinking about shifting my focus when RTL is done, to fix the colors issue, there might be a very interesting solution.