All of your textures are converted by PyPrp to
DTX format at export, so do not think you are saving any filesize by using jpegs ...
look in the "tex_cache" folder of your age (in the dat folder) ... you will see that your exported (and converted) textures have "common" sizes ... these depend
ONLY on the
pixel size and whether they use Alpha or not ... as kaelisebonrai says, png is probably the best format to work in.
Check that textures that
do not need an alpha (transparancy) layer do
NOT have the "
Use Alpha" button checked in Blender (otherwise Prp will export it as a DTX
5 - which is a larger file size - instead of DTX
1 (smaller)... You can see which format your textures are converted to during the (first) export of your textures in the Blender console window...
.... it's worth watching that window at export ... many times I have spotted textures being exported as DTX
5 that did not need to be (because I had forgotton to
uncheck the "Use Alpha" button - which is "on" by default ...

)
Remember, that if you change any texture's atributes
but retain its original name that you will have to
delete it from your texture cache - and "reload" into Blender - before exporting (or PyPrp will continue to use the "old" version from the text_cache)
The real "trick" is to re-use materials & textures (or parts of them) over and over again ... in such a way that the player does not notice ... (Cyan does this all the time)

... and, as diafero says, you can try reducing some of your 512 x 512 textures to 256 x 256 ... you might be surprised at how little difference there is in game (depending on the object's position releative to the avatar and camera).
when it comes to Age creation ... "DOH" seems to be my middle name...