It's a lot smaller if you exclude the buffers/cache (which you'll get if you read the second line of the "free" command):
Originally Posted by birdie
If you really want to compare memory including cache, run this command to flush the disk cache before logging in:
383688 - 3928 - 101748 = 278012
576068 - 30376 - 199300 = 346392
Difference: 68380 = 66MB
386824 - 4660 - 10475 = 371689
1073012 - 24796 - 605136 = 443080
Difference: 71391 = 69MB
After logging in, use each app for a few minutes before checking the stats so that it has a chance to reload whatever it actually needs.
su -c "echo 1 > /proc/sys/vm/drop_caches"
Another thing to keep in mind is that if you run KDM 3/4, some of the KDE libraries may already be loaded which will throw off the before/after logging in stats. Either way, it's not like KDE4 is using 400MB more than KDE3.