If you are using less oestrogen, then in theory you should get lighter bleeds because the oestrogen won't be building the womb lining so much.
Hurdity would be able explain things better but, my basic understanding is: during our reproductive years, the oestrogen builds the lining ready to receive a fertilised egg, then the role of progesterone is to prepare the womb for pregnancy after ovulation and so if a fertilised egg hasn't taken, the progesterone level drops dramatically thereby triggering the shedding of the lining. The sequential HRT regime mimics this cycle.
I have probably explained this in totally the wrong way but essentially it's oestrogen that builds the lining. DG x