この場合、次のような言い方ができますよ。
ーShe has gained a lot of weight, but doesn't want to accept the truth, so she wears clothes that are too small for her.
「彼女はかなり太ったのに認めたくなくて、彼女には小さすぎる服を無理して着ている」
to wear clothes that are too small で「小さすぎる服を無理して着る」
to gain a lot of weight で「体重がかなり増える・かなり太る」
ご参考まで！
Although it's not easy, I wear clothes that are too small for me.
It's unnatural, but I try to wear an outfit that is too small for me.
ご質問ありがとうございます。
① "Although it's not easy,"＝「簡単ではないけど、」
"I wear clothes that are too small for me."＝「私には小さすぎる服を着る。」
② "It's unnatural,"＝「不自然だ、」
"but I try to wear an outfit that is too small for me."＝「でも、私には小さすぎる服を私は着ようとする。」
ご参考までに。
カナダ 
アメリカ合衆国 
