Brandy Melville has fed stockpiles of single-sized clothing to young girls since 2009, featuring an inventory of plain “baby tees,” low-rise denim, tight thermals, and micro shorts.

If you’re a teen girl, or have shopped with one lately, you’ve probably heard of Brandy Melville, the Italian brand that came to the U.S. five years ago, and has lately enraptured teens ...