Comfortable thumb reach? I don't think so.

If anything, I'd say it was chosen because the size 'feels' about right.

For me the SGS II is way too big; it doesn't fit in my pockets comfortably and it feels awkward when holding it to my ear.

There's a balance - bigger is better in terms of a touch interface but smaller is better in terms of convenience. Somewhere around 3.5" seems about the best compromise for a smartphone.

For me at least.