I made a pretty big starter the other day. I made it much larger than normal because it was the first time I was using washed yeast so I put a lot of yeast and wort in there. The krausen on the starter did get pretty high and I was a bit worried about blowover, but it didn't happen. This was at about 70 degrees w/ 1056 yeast... In hindsight I probably used way too much yeast and way too much extract in the wort so under "normal" circumstances I don't expect it to happen again.