Wow, I'm 5'9" and 119-120. If your GF was 5'8" and around my weight, she has no tits or ass... She's a STICK. Damn, that's just sad.
Pot, meet Kettle.
Go eat a fucking sandwich. For the record, I'm 160lbs at 5'6" and 12% BF. Overweight by BMI, low healthy range by BF%.
http://www.cockeyed.com/photos/bodies/411-120.shtml
Wait... 120lbs and UNDER 5'? She must be a fatty!

Just proof that ass/tits can add a little weight, and that just being toned thanks to a little muscle mass can skew the scale up even more. A curvy girl can EASILY have an extra 10 pounds between her ass and tits. A pair of DDs alone probably adds close to 5 pounds. There's a reason girls with big tits complain about back pain.
People need to realize that fat takes up almost 20% more volume than muscle by weight. A girl who is 130lbs and 33% BF (considered overweight) is noticably bigger than one who is 130lbs and 20% BF (which is considered just BELOW the healthy range for women). Part of this is because of fat distribution on the body. The athletic girl will have a more even distribution of fat, and will have a nice toned looking body thanks to an extra 16lbs of muscle in place of 16lbs of fat the other girl has. The sedentary girl will have most of that fat in her hips, thighs and stomach and much less muscle mass making her look lumpier.
Basically what I'm saying is... Pics or GTFO.

Otherwise it's almost impossible to pass judgment unless someone is WAY out of the normal range.