Neither of these sentences is correct English. Neither 'on condition' nor 'in case' works in this context. The sentence you need is this: IF you ate less, you would not get so fat. This is a standard 2nd conditional construction.

They both mean the same thing but the structuring of these statements do not sound perfectly natural. The most correct simple version would be the imperfect conditional setup: If you WERE to eat less, [then] you would not get fat. The simple formula is: IF (subject) you/he (imperfect form of to be) were/was (infinitive verb) to exercise regularly, THEN (subject) you/he (conditional) would look healthier. If you were to exercise regularly, then you would look healthier. If he was to exercise regularly, then he would look healthier.