I saw the comment and wrote it off as incorrect. The only wake encounters I have had in a 747 have been from other widebodies. I think the big cause of wakes are wing area and wing loading. As one goes up, the other goes down. It's only when you have a large wing area and a large wing loading that wake danger intensifies, so you need a large heavy aeroplane- big though the 757 is, it just doesn't cut it against a heavy 747, and it has a large wing reducing wake problems. Simplistic, but I think it all simply boils down to weight. But I will now be corrected..............