Huge, Bulky, Enormous, Extensive
noun, な adjective, の adjective
A thing that swells big ends up being huge or at least really bulky.
ぼうだい
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
これからこの膨大な資料に目を通さなくてはいけないんだ。
You have to take a look at these huge documents from now on.