Why Lightroom reduce image quality after export

Hi,

I'd like to know why lightroom reduce the quality of my photos after export?

I use RAF as orignal format, when I export in JPEG, my photos seem to lose the details. And all JPEG images only have 2-3 Mo as size, while the original ones usually have 20-30 Mo for each photo.

Where could I set the quality for exporting? 

Thank you!

Yvonne