She looks fine - remember it is not the size of the belly you should be checking anyway, as that can vary from day to day according to how stuffed with fibre the horse is. The crucial areas to check are crest of neck, ribs and quarters. Ribs should be easy to feel but not visible.
if you can easily easily feel her ribs shes fine.
you dont want a young horse to be fat but as long as its grass belly (and shes not got a worm burden as this ca cause a pot belly) shes fine.