The dancers look delicate in their white dresses, but in reality they're as strong as horses.
Literal
Dancers [topic-は] white dress [object-を] wear-[and-て] delicately is-visible-but, in-reality they [topic-は] horse [as-のように] strong is-[explanatory-のです].
A two-clause sentence contrasting appearance with reality. のように ('like / as if') is the simile marker — 馬のように ('like a horse'). 実際には ('in reality') is the factual contrast with the visible appearance. きゃしゃ ('delicate / slender') is a native-Japanese adjective. The whole sentence neatly balances visual impression vs hidden strength.