Her tits are smaller because;
1. She got older. If a chick leaves her late 10s, and starts her early 20s, the breast-skin is not that tight anymore, and as time passes by, they got a little hanging, but if she is fortunate, the shape remains, but the skin loses tightness. That is fact no. 1.
2. She has lost some weight as well. If a chick loses weight, they lose it from several frequented "fat" areas such as; breasts, stomach, ass, and thies. She has lost weight, she has lost fat, so she has "lost" some of her previously "fat" tits. That is fact no. 2.
These thing happens to all chick in the world.
If sshe would gain some of her weight back, the tits would get fatter as well, but it wouldnt be the same shape as it was when she was younger. They would be fatter, but a little more "hanging" down. Sad, but true.
Satisfied?