Yes - reduce "message to sync" and "messages to cache" settings and then Menu -> Compact the database
The defaults in app settings (under mail, receiving) are 50 and 250 - which means "look at" 50 most recent messages on a mail server (per folder) and also keep 250 stored on the device in case the user scrolls down a message list (so they don't have to be reloaded).
There are also per-account options that override app settings if set (long press on an account -> options and folders -> account options).
I'm guessing you might be using much larger values so going back to 50 and 250 would be first step (and doing Menu -> Compact the database after that).