I think the reason why the changes are more successful for FTMs is because being a woman is the default state for the human body. This is seen in those XY people with testosterone insensitivity, whose testicles do not descend and appear phenotypically female. Genitalia and a little breast growth excepting, men and women share approximately the same bodies until late puberty where male changes cause a divergence from this. As a side note, this is a significant contributing factor to how MTFs may look younger than their age; they don't get the bone changes if they start hormone therapy later than this as the bone structures are set. They have the other changes and presentation of a male, but the bone and body structure of a younger male.
When one takes T as a person with female physiology, one instigates those changes. When one takes E and suppresses T one is trying to reverse them. Its clear to see that one is more successful than the other. As the one major reversal in the ftm camp, take breast growth as an example; The E instigates the change and breast growth is relatively successful in MTFs. The reversal of this is less successful and though one might lose some fat, the breast growth is largely irreversible without surgery. In MTFs, the changes that have been instigated by the T are facial bone structures including brow ridges, nose width, jaw etc. bone structures in the body making the body generally taller and wider with bigger hands and feet, increase in muscle mass, hair growth and thickening, thickening of the vocal chords, formation of the adams apple, male pattern hair loss etc. Some are partially reversible with hormones but most require other interventions.
As the male condition is a divergence from the natural female state, for MTFs there are fewer changes to instigate and more changes to reverse. As a FTM there are more to instigate and less to reverse. This is why they have a harder time.
