There's a lot of turbulent airflow between the ET and the Shuttle -- it's possible that something big -- ice or insulation -- could have been dashed directly against the tile, and not been just a glancing blow.
The blob, and the size of the "puff" after impact suggests that it was something fairly good-sized.
Judging from the comparison of the blob to the cabin door, it looks like the thing has dimensions on the order of a couple of feet.
The fact that it was pulverized suggests that there could have been a good sized impact. (Or it could simply have been pulverized by shock waves -- like I said, lotsa funny stuff goes on in that area.)
FReeper John Jamieson has posted info on testing the tiles by dropping quarters on tiles. John, do you have any insights as to what a 2-foot hunk of ice or insulation could do to, say, the main gear door?