Doctor said I had a slightly hypoplastic labia majora. What does this mean?

Smaller than others' "Hypoplastic" is the opposite of "hyperplastic". "hypo" means "under", "hyper" means "over", "plastic" means "built" - so hypoplastic labia would mean smaller than "normal" labia, hyperplastic would mean larger than "normal" labia. There are no reference values for what constitutes "normal".