small letter = interview/rejection/offer/acknowledgment/whatever they fancy to get you scared large letter = interview/rejection/offer/acknowledgment/whatever they fancy to get you scared
It most probably differs from Uni to Uni but for L'pool last year I got a small envelope and it was a rejection - it contained just one piece of paper. For UEA this year I got a bulky envelope and it was an interview. Kinda makes sense - they give a form to send back, a map/directions etc so the letter's bound to be bigger.
It is not true in the slightest. A lot of universities don't send you a rejection letter and those that send you an offer or an unconditional often don't send the information pack,about the course, right away.
So,a thin envelope may or may not be a rejection but could be an offer or an unconditional.
I got my Durham offer in one of those white envelopes that are about 1/3 of a piece of A4, and it only had a two sheets of paper, I think. I was so convinced that it was a rejection that I read it (skimmed it) and showed it to my mother saying 'Well that settles it, I'm going to Sheffield' and she was just 'o_O?! But this is an offer...' As it happens I went to Sheffield anyway, but that's by the by.