The "dimensional-anchors.log" file is exactly what it's extention hints at a log of every time the Dimensional Anchor checks to see if a player is on the server. The reason the file gets so big is that after just a few seconds it will log 6kb of entries! It will create absolutely enormous files in very short order (1mb every 2.8 minutes, and 1gb every 2.25 days).
The problem is that this constant log is pretty much useless, as I can't think of any reason I'd want to be able to open up the file and see the exact second one of the worlds was loaded (which is 40 times a second -- fyi)...
I'm currently trying to find a way to disable this pointless file, and if you also think it's ridiculous having this file bog up our hard-drives, contact the mod creator here (remember to be polite, he does make these mods for free and they are rather labor intensive)