Cada vez estoy más gordo
 

to translate cada vez en está frase sería every time o all the time.

I'm growing fatter and fatter all the time / every time. Every time sounds out of place to me.
Anybody help !

Both can be used. Saying 'every time' seems to gives emphasis that there's a time or place where the observation is more pronounced.

great! thanks

I keep getting fatter, is a very natural way of saying this. For help I would suggest going to a gym and spending at least a half an hour each day on a rowing machine. You will keep getting thinner:D.

Cada vez estoy más gordo.
I keep gaining weight.
I keep getting heavier.
I'm steadily gaining weight.

Remember that while fat = gordo in a literal sense, in usage the meaning is a lot more nuanced. Culturally "gordo" is not nearly as offensive in Spanish as "fat" is in English. I tend to translate "gordo" using heavy or weight when possible, unless it is clear that it is intended to be a direct insult.

"mi gordito" is a term of endearment and should never be translated "my little fatty" which would be offensive/insulting.

EDIT: "I keep getting fatter" is fine, especially since you're talking about yourself, but is more informal, and would be insulting if talking about someone else.