When apps target users in developing countries, there is pressure to keep apps small. Those users tend to have phones with less storage and tend to sort by size when deleting apps to install something else.

Some users will build up a cycle of delete A and B to install C, then delete it to put back A and B. The smaller your app is, the more likely it is to stay installed because it's not as helpful to delete it.

The biggest obstacles are small storage phone users are well represented in non-Play store installs, which often means a fat apk that has binary libraries for all platforms and raster images for all dpis. The other big issue is translated strings; Android standard string translations can't be compressed (apk is a zip, but not everything in a zip is compressed), even though they would be very compressible. There's a lot of languages out there, and not using the standard translation tool is hard to sell (and you still need it for somethings anyway).

Doesn't help for US based banks though. Mostly their customers are going to have enough space that a bank app isn't going to get deleted to make space for whatever else.