That would be me:
Spank Slaves TGP
I came up with that at a time back in early 2002 when I had never even seen a thumb tgp -- I was so clueless I thought it was my own idea. So I can't claim there was any thought process behind choosing that size, it didn't matter much on a tiny hand-crafted TGP that didn't trade traffic and got a few hundred hits a day.
Don't know if I'd choose 150x100 again knowing what I know now -- it gets pricey in bandwidth -- but it's making me money so I'll stick with it.