I've seen how mayonnaise is made.
It's basically 1 eggs, 1/2 a litre of vegetable oil, with a bit of mustard, garlic and salt.
Then it's just whisked up
When you put this on your food it is about 80% vegetable oil.
If you're a bit on the heavy side I wouldn't recommend it at all, unless you're trying to put on a bit of chub.