Um, there is some terminology confusion here.
To an Android developer, Internal Memory is where apps are installed. Most likely not 8 GB, even on very shiny and expensive phones.
In that memory, Aqua only stores metadata and message text.
External Memory aka "memory card" -- as called by Android -- is the "logical" external memory. Used to be a microSD a few years ago, now typically built-in. Logically, it's different, even though it may be a partition on the same memory chip. This is your 8 GB partition. This is where Aqua stores attachments and inline images.
The "real external microSD memory card" -- the third thing -- is not used by Aqua. In theory, I could store attachments and inlines there, even with 4.4 (apps are allowed access to their own directories, even on the "real external microSD"), but there has not been any work for that.