Big things such as animals (swimming next to whales) transportations (huge airplanes at the airport, submarines out of water, huge ballons at the garages, Huge Ships whatever they are but especially when they're underwhater), Also I get this creepy feeling when I'm using google maps and I zoom out and see huge continents/countries.
It's the wierdest phobia ever but I used to have nightmares about it
Human beings are complicated
EDIT:
I just found it ! It's called Megalophobia, it's actually a very common thing
This the first time I'm reading about it
EDIT 2:
I also have Claustrophobia too