big top vs round top

big top and round top both are nouns.

Word	Noun	Adjective	Verb	Adverb
big top	Yes	No	No	No
round top	Yes	No	No	No

As nouns, big top and round top are synonyms defined as:

big top and round top: a canvas tent to house the audience at a circus performance

Other synonyms of big top include circus tent, top.

big top (noun)	round top (noun)
a canvas tent to house the audience at a circus performance	a canvas tent to house the audience at a circus performance