About the only negative feature of MBAM (I've got the free version) is that each update reloads the full defs database, currently pushing 6 megs. Supposedly they were going to convert to an incremental-update system, similar to avast's, sometime last summer, but I haven't yet seen that or heard anything recently about it. If I check Disk Cleanup, it apparently leaves behind about the same amount of temporary internet files (possibly a backup copy of the old defs?), which SAS also does, both of which can be safely cleaned out.
On the plus side, they've made a huge improvement in the scanning engine, and a quick scan now takes about half as long as it did say a year ago, maybe 2-3 minutes now on my system.