The quick answer- it doesn’t have to be! It’s true, disposable diapers will always have the advantage in the “trimness” category. But there are several cloth diapering options that will save on bulk.
Choosing a natural fiber diaper (cotton, bamboo, hemp) will offer more absorbency per square inch than a synthetic fiber (microfiber), allowing for a trim diaper without sacrificing absorbency.
Also consider a sized diaper, rather than one size. One size diapers will fit from approximately 10-35 lbs., so there can be quite a bit of extra material, especially on a small baby. Sized diapers will reduce bulk caused by extra fabric.
A side snapping diaper, like the GroVia AIO or Peachy Green AIO, create a smooth front panel, and eliminate bulk around the belly.
WIth that said, in my personal experience, cloth diapers alone have never been a reason to size up in clothing! If you are still worried about the bulk, there are a few other options for you. Baby Legs, essentially a baby leg warmer, eliminate the need for pants at all. Wool longies or shorties will also eliminate the need for pants, serving as both a diaper cover and clothing.