round top vs big top

round top and big top both are nouns.

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

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

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

Other synonyms of round top include circus tent, top.

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