I agree with Dumac, and I also partially agree with a previous poster that said that if you care about 5 lbs, you may be too shallow for the relationship.
5 is not much and can come and go (I'm a fat skinny guy that's 5'6 and weighs 125), but if she gains 10 pounds that's quite a lot.
Also, if you're that worried about her weight and health, you had damn better be too taking care of yourself too. Otherwise, you're just shallow and have a sense of entitlement.
Anyway, if you really think shes unhealthily gaining weight, try working out together. Go swimming together or take hikes. Also, there might be underlying reasons (depression, stress?) for the weight gain.
If I wanted my gf to lose weight, I would become a lot more active (gym, hikes, swimming, etc) in order to encourage her to participate and lose weight.
They say that the easiest way to get your SO to exercise is by setting an example yourself.
That said, five pounds is not a lot of weight.
Also, 5'5 and 110lbs sounds way to stick like for my liking. It is also technically underweight.