Perk, take it as I'm saying it, respect the woman enough to say to her, "I think you're a wonderful person and would hate to do anything to ruin our friendship but I respect you far to much to lead you on in any way, you're just not the physical type I go for." If she asks what you mean, then tell her you go for slimmer ladies. That way you're being dead honest, the decision as to whether to lose weight or not is entirely up to her and she can also decide how she feels about how much you value appearance. She might think you're too shallow and will just back off. Either way, problem solved.
It's been my experience that trying to sugar coat honesty backfires in the end.