Remember that 저 means "that over there." 저 can also mean, "that," but its more common to use "저" for "that over there."
Dont get confused with "물건" and "건물" Since the words are swapped, it can make the two nouns confusing. So you have to remember, that "물건" means THING and "건물" means BUILDING.
And if you remember from Adjectives 1, "큽니다 " means big.
The subjects of the sentences are different among other things.
"That building is big." has "that building" as the subject of the sentence and the sentence is a description of the building.
"That is a big building." has "That (thing)" as the subject of the sentence and the sentence is equating "that (thing)" with "a big building".
Not a huge difference, but there is one.