Macnair was tall but not thin at all, he was massive with muscles, remember? Though I suppose he might be thin after a year in Azkaban. But I guess that Snape was always thin, and a bit tall, too. And if he's going to be seen anywhere, it'll probably be where he can get at Harry, so I think you did the right thing to speak up.

Anyway, don't feel bad about Mr Gupta being stern, it's something Aurors do all the time, they play Good Auror/Bad Auror. One comes in and acts fearsome, and then the other comes in and acts apologetic and kind. It's meant to make you trust the kind Auror so that you'll be more likely to tell them everything they want to know.