20 mg/day Anavar is 140 mg/week, and is comparable in anabolic effect to even more testosterone than that.
For reference, 140 mg/week is a full hormone replacement therapy dose for a man. Of course you're putting on weight and muscle rapidly.
Searle originally dosed oxandrolone at 2.5 mg/tab for a reason. It was an effective anabolic for woman at this dose and had relatively little virilization problems.
The higher dose 10 mg/tablet Oxandrin brand was medically introduced for AIDS treatment (seriously, by BioTechonology General) but this dosing for women commonly leads to virilization.
10 mg at a time is entirely suitable for today's female bodybuilding. Not for bikini.