higher dosages of HCG allegedly lead to estrogen spikes because a higher signalling of HCG, which is a LH analogue (or FSH ? i always confuse the two) leads to more testosterone production, however a certain portion of testosterone is always aromatized into estrogen...
the theory is that theres a limit to the amount of testosterone your body can produce in response to HCG and that big dosages lead to excess estrogen but not any more excess testosterone production than you would get with more sensible amounts
since you actually only need 250 to 500 iu once or twice week of HCG to mimick natural testosterone production (there are studies on that, i think someone recently posted them on here or promuscle) then using a much higher amount at once would probably lead to massive estrogen production
WHICH IN TURN would suppress your HPTA further and actually prevent recovery... excess estrogen = shut down