There's two core factors as I understand it. First beauty norms are very gendered and center heavily around weight and body composition. There is a higher pressure on women to be "thin" (denormalize that idea), and it does inflect how we as trans people relate to our bodies.
Second, fat distribution is controlled by sex hormones which means it is alterable. Once your hormones stabilize, body weight fluctuations disproportionately affect 'incorrectly' stored fat---to different degrees for different people.
Losing weight during HRT, particularly for transfeminine people, is a double whammy since you are meeting gendered beauty norms AND altering a secondary sex characteristic.
It is a completely valid thing to notice and feel euphoric about! At the same time, don't overextend it into fatphobia or self-hatred. Let go of internal fat shaming because gaining weight now will probably turn you into an ass monster! ;)