Questions & Answers about Milijun je velik broj.
Because je is a clitic in Croatian. Clitics are short unstressed words that usually go in second position in the sentence or after the first stressed phrase.
So in:
Milijun je velik broj.
the first element is Milijun, and je comes right after it.
This is very normal in Croatian:
- Ana je ovdje. = Ana is here.
- Ovo je dobro. = This is good.