round shape vs bulb

round shape and bulb both are nouns.

Word	Noun	Adjective	Verb	Adverb
round shape	Yes	No	No	No
bulb	Yes	No	No	No

As nouns, bulb is a hyponym of round shape; that is, bulb is a word with a more specific, narrower meaning than round shape:

round shape (noun)	bulb (noun)
a shape that is curved and without sharp angles	electric lamp consisting of a transparent or translucent glass housing containing a wire filament (usually tungsten) that emits light when heated by electricity
	a rounded part of a cylindrical instrument (usually at one end)
	a rounded dilation or expansion in a canal or vessel or organ
	lower or hindmost part of the brain; continuous with spinal cord; (`bulb' is an old term for medulla oblongata)
	a modified bud consisting of a thickened globular underground stem serving as a reproductive structure
	anything with a round shape resembling a teardrop