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For instance, a map tile is a png image with a size of 12.6KB.
In the image below, the map loads 9 map tiles (= 9 map images) at least. This means, drawing the map below will download 113.4KB (= 12.6KB * 9) of image data.
You may not feel inconvenience about the map when you're browsing using your desktop browser, because your desktop environment has much more memory and faster a CPU than your mobile device.
The value of 113.4KB is a lot of data for a mobile device. In generally, the mobile network speed is less than WiFi internet, and your device has less memory and less CPU power than your laptop. It means loading map tiles takes a few seconds. If you zoom in the map, the map will load 113.4KB again.
Google Maps mobile SDKs
Google provides Google Maps Android API v2 and Google Maps SDK for iOS. They load only map data for drawing map tiles, then the SDK draws the map. It's probably less than 12.6KB. (vector data)