"Please pardon my ignorance if this is a silly question, but if lowered estrogen levels translates in serious fatloss in women, wouldn't menopausal women be all slim like some anorexic french models?"
It would be if sex steroids were the controlling agents in bodyfat mass, but they are only small players in this topic. The fast weight loss was most likely due to alterations in Na+ retention, which estrogen does modulate.
Another problem with the concept of estrogen controlling obesity would be the idea that most women have their best figures during their highest estrogen times, teens and 20's, with increases in fat mass as they age (where estrogen levels often decline).
What many use anti-estrogens for would be to aid in the spot-reduction of estrogen-sensitive fat depots, such as hips and thighs. Abdominal fat stores are actually reduced by estrogen, since it causes an increase in lipolysis in this area, hence the "hour-glass figure".