I truly do not intend to be crass or unsympathetic about this subject, but I don’t think it’s unfair for a boy to wish to marry what he considers a good-looking girl. And yes, the slimmer, the better (if this is something important to him, and if that is what he considers a factor in looks).
Does a girl have the moral right to decline to marry an otherwise perfect boy because he has an unsightly lump on his nose? The answer is a resounding yes! I think the boys are subject to the same moral equivalency. They have the moral right to turn down a girl who they deem too heavy for their liking.
Now, you do raise a valid point as to why ehrliche bochurim would care about looks etc. (or why ehriche girls would care about looks as well, for that matter). However, as long as our boys and girls are not trained to ignore looks as a factor in marriage, it would be unfair to expect anyone to look away and ignore this. After all, heavy girls (and boys) are not as physically attractive as their thinner counterparts, and we would be living in denial if we think otherwise.