If it's particularly rich, like alfalfa(which I'm guessing you're not, by what you've said & the way she looks!), then I would up it gradually, but if it's just grass hay, because you feel that she might be going hungry, I'd just go ahead & provide her 'quota'. **If she has been getting hungry before meals & then you give her free choice, she might 'pig out' for a bit, but they do often settle down soon & stop gorging, when they realise it's not going away. But of course, be nice to think that were always the way - some of them NEVER know when enough's enough!
Slow feeder nets or such are good options for that!