Questions before treatment

Letrozole

Hi, I am 43 and a little overweight. I already have 3 kids and we want to try again with a baby. With the other pregnancies I always stimulated with menogon and then gv as planned. I respond pretty poorly to meds, have always injected 150 units of Menogon for a follicle. My doctor said that 2.5 mg of letrozole was enough and that taking 5 mg of letrozole wouldn't help anymore because I was still having episprudences. Can you explain to me why it is not better to take 5 mg of letrozole, then I might have more follicles or better ovulation. Or are there downsides to taking too much letrozole. Unfortunately, my doctor did not explain anything to me dZu. Thank you