I bought not only a Christmas tree but also lots of presents.
What did you buy?
"bought" is a transitive verb that takes an object
I bought a Christmas tree. [tree = noun = direct object]
I bought lots of presents. [lots = plural noun = direct object]
See Definition 5:
http://www.merriam-webster.com/dictionary/lot
Parallelism is achieved with the direct objects being of the same form: nouns
I bought not only a Christmas tree but also lots of presents. (noun/noun) NOTE: It doesn't matter if one noun object is singular and the other plural; they are still of the same form: a noun.
I / bought / not only / a / Christmas / tree / but also / lots / of presents
Subject, pronoun / verb / conjunction / article / adjective / noun / conjunction / plural noun / adjective prepositional phrase that is modifying "lots" to describe, as an adjective would, "What kind? "Lots of what kind of things?" Presents
16a. Express parallel ideas in same grammatical form. Coordinate equal
rank and connected by and, but, or, nor, or:
16b. Place correlative conjunctions immediately before the parallel
terms (both..and, either...or, neither...nor, not only...but also)
John E. Warriner. Warriner’s English Grammar and Composition. Fifth Course. Liberty Edition. Orlando, Florida: Harcourt, Brace, and Jovanovich. 1986. 303-08.