>>149671439
The whole point of the Spoiler costume was to hide her identity (she was also younger and probably less developed when she started). Tim thought she was a short guy at first before he took her mask off and was shocked by being pretty. Even the big Spoiler shoulder strap seemed to be made to cover up one of her breasts in the first few issues she was in before it got shrunk.
After a while you started getting people drawing her more busty in the suit. But Robin in the 90's was still considered a book meant to be sold to kids so they probably didn't want to make her too sexy.