Well each replay takes up on average 50kb, so after awhile yes. I could see the autosave feature becoming a bit of a bother.
Take Hamster for example; 13,000 total matches.
If all autosaves still there. (averaging on 50kb a match ( sometimes more sometimes less)
13,000 x 50 = 650,000 KB, which is 650 MB, which may not seem like alot. but that space can be easily missed in the future D: