？
Where on earth does all that fit in that skinny body of hers?
Literal
That skinny body [gen-の] where [to-に] enters [question-んだ]?
細っこい is a colloquial/emphatic variant of 細い 'thin / narrow / skinny,' with the middle consonant geminated and the ending softened into -っこい. The diminutive-sounding ending conveys 'very skinny / comically skinny.' どこに入るんだ '(I wonder) where it enters' is the rhetorical question expressing astonishment that such a large amount of food (or whatever) fits in a small person. ~んだ is the assertive explanatory.