It's a reason, not an excuse; there's a difference.
I have a 2 Pound coin that I use and it just doesn't work. I don't have a half dollar so that's not an option. I have what I have and it just isn't big enough. If my thumb muscle was bigger, it would make a smaller space thus making it a perfect reason and therefore, once again, I win.
The only difference is that an excuse is used to avoid a negative consequence, like receiving the advice that you can't perform the move because you haven't practiced enough, instead you use the excuse
that there is a problem with your hand size. We'll use your word, though.
Two things are wrong with this reason. First of all, if you practice, your thumb muscle gets bigger. David Stone has an exercise for your pinky muscle as well on his Basic Coin Magic DVD's, if you're interested. Any exercise that works your classic palm would improve your muscle pass as well (same or similar muscles). Second, if you got a bigger coin (or, say, a Poker Chip), you could practice and improve your muscle pass. You can get things of the right size to practice anywhere. Bottle caps (various sizes), poker chips, coins, whatever! And for your information, English Pennies are the same size as a U.S. Half. Both are sold by Penguin, along with gaffs, Chinese Coins the same size, and plenty of other things you can use to muscle pass. No excuse. I'm sorry, no reason.