The zoom is calculated automatically by Google using their ‘fitBounds()’ function. That function tries to set the maximum zoom that will still show the region in question. The zoom is based on the rectangular bounds of the region to be shown and the size of the map container.
Given the same map size, a very small bounds (like a single street) will zoom in close, while a large one (like a country) will zoom further out. I think in your case Pattaya is probably a little smaller than Bangkok.
There’s one other case as well: for a map of locations without defined bounds, such as a single street address, the default POI Zoom is used from the MapPress settings – but that’s not the case in your examples.