The size at which a designer presents their logo is pretty much up to them. It's an individual aesthetic, for the most part -- some favor larger margins, some choose to make their logo as large as possible within the space allotted. For the most part, I think we all tend to fall into an average range, but there are definitely some folks who lean towards one extreme or the other. If there is a design you like, but want to see larger, you could possibly leave a request on the designer's submission page and ask them to resubmit with smaller margins (or, vice versa).