tidy sum vs heap

tidy sum and heap both are nouns.

Word	Noun	Adjective	Verb	Adverb
tidy sum	Yes	No	No	No
heap	Yes	No	Yes	No

As nouns, tidy sum and heap are synonyms defined as:

tidy sum and heap: (often followed by `of') a large number or amount or extent

tidy sum (noun)	heap (noun)
(often followed by `of') a large number or amount or extent	a car that is old and unreliable
	a collection of objects laid on top of each other
	(often followed by `of') a large number or amount or extent