If you can easily palpate her ribs (maybe 1/4 inch of skin and fat over it), her hips are only slightly boney, you can feel the tops of the vertebrae easily, and she has nice firm muscles, she may be at an ideal weight (a sort of long & lanky build). I have a civvie who feels like that and despite food beeing available, she doesn't eat more.
On the other hand, if she has lots of padding and it is difficult to feel the ribs, hips, and vertebrae, much less the muscles, then you might cut back 1/8 to 1/4 of a can less at each feeding.
OR, regardless of weight, you might try active play sessions for 5-10 minutes several times a day. The play sessions may help with the glucose levels too (at least activity helps in human diabetics) and support the bond between you.