it may still download emails to the phone. if you go into application manager in settings, then look at the size of the gmail storage app, that will tell you how much your emails are taking up on the phone (just make sure that you change the filter to "all" instead of "downloaded" if it has that option. if you clear this data it will delete all the emails, so better to do it through the app if it is taking up a fair bit of space. also worth checking the sent items part of your emails in the app if you have sent any emails with attachments, as that can chew up space a bit.
for you to have so little space left either the new rom must have a lot of extra crap in it that you don't need, or there is something taking up a lot of space. may be worth looking to make sure you don't have stuff that doubles up.
also when you are in the apps manager, you can sort the apps by size, so you can have a look at the big ones and see exactly what is taking up the space