It's hard to measure growth with such animals: the oral disc and tentacles can be expanded and contracted at will.
Indeed, in certain cases, underfed animals will expand to get more sunlight, so that hosted zooxanthellae can phtosynthesize more starch and sugar for the animal.
I wouldn't cut back on feeding at all, no.
Enjoy the animal.
If you like, you can find a suitable clownfish to take up residence in it --the constant disturbance by the clownfish tends to restrict anemone expansion somewhat. The mess the fishes make when eating and especially when defecating, provides nutrition for the anemone.
Suitable Amphiprinids for your Macrodactyla doreensis (LTA)include:
Yeah I have a clownfish that hosts in it. Nemo was just eating when I took this picture. I've been feeding it 3 - 4 times a week large chunks of shrimp. It really started growing when I was feeding it so much so I've tampered back on feeding it so much.
It has it's foot anchored in the sand right under the rock. When I first got it I dug out a small hole right at the base of the rock and placed it in the hole and pushed sand around it. It has stayed there without moving at all.
I'm just afraid it's going to outgrow the tank now
Your fear is not unfounded, ...hehe...
I've kept a few M. doreensis (most handed over to other aquarists, and a few returned to the precise reef whence they came. They can all take up quite some space!