It's not that bad, depending on what you're doing inside of it. My password manager clocks in at ~150 MiB RAM usage with a few thousand passwords. Could be better, but it's not as bad as, say, Mozilla products written in XUL.

A password manager shouldn't even need a tenth of that.

Realistically? 15 MiB is going to be hard in any desktop GUI toolkit I know. Something around 50 MiB is more what I'd consider feasible.

"a tenth" of "~150 MiB" is a reasonable approximation:

    $ ps -p $(pgrep keepassx) -o rss=
    22844