1. to fit = ser de la talla de alguien
2. to lose weight = adelgazar
3. to put on weight = engordar
4. to be on a diet = estar a régimen
5. to weigh = pesar
6. to cut down on = reducir
7. to do exercise = hacer ejercicio
8. to give up (smoking, drinking) = dejar (de fumar, de beber)