Literal translation
make yourself fat!
Meaning
enjoy yourself, indulge
Explanation
The starting image is food: you are given far too little, and, ironically, you tell yourself with that, fais-toi gras, fatten yourself up. The phrase points out that what you were given, or told, is insufficient or useless, and later spread to other situations, such as reporting incomprehensible explanations to a friend. It works through ironic antiphrasis, a common device in Provence. Some add vaqui uno amelo or aqui un cese depending on the region.