You are correct that it seems that the Pacific is covering half of the Earth and the landmass cover the other half. But in Earth's history there were several times that it was even more extreme. From about 250 to 200 million years ago, all continents were concentrated even more in a supercontinent called Pangaea:
This continent would cover anything from a quarter to a third of the Earth's surface, the rest being a massive ocean called Panthalassa.
The reason this happens is plate tectonics: lithospheric plates, which carry the continents, move and drift slowly. Here's a nice simplified animation of how this works:
Another animation showing how this changed during Earth's (recent) history:
As for now,
Is this coïncidence or plays there something else?
This is not a coincidence. The oceanic spreading centres that appear in the above video spread at different rates. The one at the Pacific does so relatively fast, and a large mass of oceanic lithosphere is being constantly produced in there. Consequently, the Pacific is quite large. It is slowly getting smaller though. Today's Pacific is the remnant of the old Panthalassa which used to be much larger. The Pacific is surrounded by subduction zones that recycle the oceanic lithosphere back into the mantle, resulting in the infamous Ring of Fire around the Pacific ocean.