You can say something pke:
"Wow, you look really fit!"
"I'm impressed by your dedication to fitness."
"You must have been working out a lot. You look great!"
"Your muscles are looking super toned. Keep up the great work!"
"You have such a fit and healthy body. It's inspiring."
It's always a good idea to be sincere and specific when comppmenting someone on their fitness, as it shows that you really appreciate their efforts and the results they have achieved.