You are really skinny to begin with, so gaining a bit of weight would be absolutely no problem.
"You are really skinny to begin with"＝「君はもともとすごく痩せてるよ」
"so gaining a bit of weight"＝「だから少し体重が増える」
"would be absolutely no problem."＝「は全然問題にならないよ。」
☆その後に、"You would still be very skinny after gaining a bit of weight."（ちょっと体重が増えても、まだすごく痩せていることになるよ）と言えます。
アメリカ合衆国 
