You are in dazzling shape! What are you doing?
'Smagliante' = sma-gLIAN-te, stress on second syllable. 'Forma' = FOR-ma.
Use this when someone looks noticeably healthy, fit, and energetic. 'Forma smagliante' is an enthusiastic Italian expression. Following with a question about their habits invites a conversation about fitness, diet, or lifestyle changes.
'Essere in forma' (to be in shape) is the standard Italian fitness expression. 'Smagliante' means dazzling/brilliant — it originally referred to the flash of a gemstone. 'In forma smagliante' is a vivid hyperbole for looking exceptionally well.
Sembri più giovane di prima!
You look younger than before!
Compliment on apparent reverse ageing — always welcome
Hai perso peso? Stai benissimo.
Have you lost weight? You look wonderful.
Careful — use only if clearly visible and you are close to the person
Hai un'aria di salute che fa invidia.
You have a healthy glow that makes people envious.
'Fare invidia' (to make envious) — used positively in Italian
Italian health culture is deeply tied to the Mediterranean diet and active lifestyle. Complimenting someone on their physical form is completely natural in Italy — there is no cultural taboo around discussing appearance as there might be in some northern European countries. Health ('salute') is considered one of life's most important values.