How to make my buttock plump and fat again?

I've started biking and working out a bit and I find although I've pretty thin, my butt has become flatter. I would like to have my plump fatter butt back. How can I make my butt plump and fatter? Are there any exercises or health tips that will make my butt plump and fatter?

I have a really high metabolism, so I sped pounds easily and find it difficult to gain weight.
How to make my buttock plump and fat again?
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