I haven't actually measured anything, I know when it first came out there was some concern about people possibly hitting bandwidth caps. I think part of the iss

I haven't actually measured anything, I know when it first came out there was some concern about people possibly hitting bandwidth caps. I think part of the issue may be overhead - it's regularly communicating back to the servers with current location, and getting back a list of coordinates for stops within a certain distance (with a bit of other data), coordinates and other data for Pokemon spawns, etc. It's also using Google Maps data to draw a map of the terrain including roads, water and building outlines, so all of that is also something that changes as you move around. Finally, people are also supposed to "spin" Pokestops and gyms, each of which also has a photo of the piece of artwork, etc. that the stop is located at. Along with the map data, those photos are not preloaded in the app so they have to be downloaded at least once. I don't know how much caching of those images is done - hopefully some, but if they're doing little or no caching then that's yet another chunk of data to pull down. Looking at the screen and a map, it looks like the game renders an outline for every structure within 600-650 meters of me, so for an area with a lot of structures that could be a fair amount of downloaded data depending on how good their caching is. If their download process isn't good at handling missing data or transmission errors, it could also be pretty vulnerable to problems on a congested network.