No, those "hills" are actually mountains. Onigashima is surrounded by a whole mountain range according to Law.
The mountains on top of the skull are 7 - 10x larger than the Numbers. The numbers are ~60m in size. This site
calced the mountains - it's a general calc about Kaido lifting Onigashima but they had to do smaller calcs for their end result - of the skull.
Also, houses and buildings are generally rather big in scale since everything is big in Onigashima.
It's one of Oda's inconsistency examples, you could see Zoro's body when he cut Pica's golem as well.
His scale drawings are especially inconsistent in Onigashima but if he purposely draws those mountains far larger than numbers, it's definitely a statement.
Because that's happening on panel lol. Sorry for analyzing what Oda draws...