I feel for you, my dd was exactly the same at that age. She would literally eat all day long and stil say she was hungry. Pinching food behind my back, the lot. She even pinched money to go to the shop. It was all healthy and she was (is) active so she wasn't fat but was totally solid iykwim. I did worry about her getting a weight problem. Anyway fast forward a couple of years and over the last year from age 8 to now (she's 9) she has settled a lot, she still has a great healthy appetite but not so constant. She's stretched in height, so although she's def not lost weight, she is in better proportion and seems comfier.
We had to address the pinching, as for me that was a separate and serious issue to the eating. We reiterated if she's really hungry then we won't deny her but the stealing had to stop, she was disciplined for that until it eventually stopped.
The eating thing seemed to settle on its own.
Hopefully your daughter is in a phase which will pass too. I get it's so frustrating and worrying x