Well, that's proportional to RES, it's the amount of physical memory used by the app at that moment (including all loaded libs and possibly shared memory). It only reflects your apps "memory usage" if the box isn't under memory pressure (i.e. too much running at once). If your box *is* under memory pressure, then RES and %MEM will relate to what the OS is running at that moment - that figure will go down when bits of your app is paged out for something else to run, even though your app is still "using the same amount of memory" (i.e. it hasn't free'd anything).

My first guess would be that they've just got a good old-fashioned memory leak.

The OS generally makes good decisions regarding memory use. The best advice is to not worry about it too much until you have a problem app, unless you're into learning things (which is cool).