Two big shifts happened in the American cellphone industry over the past year: Cellular networks got faster, and smartphone screens got bigger. As a result, people’s consumption of mobile data nearly doubled. In the United States, consumers used an average of 1.2 gigabytes a month over cellular networks this year, up from 690 megabytes a month in 2012, according to Chetan Sharma, a consultant for wireless carriers, who published a new report on industry trends on Monday. Worldwide, the average consumption was 240 megabytes a month this year, up from 140 megabytes last year, he said. But what’s in a megabyte or gigabyte anyway? A megabyte is about the amount of data required to download a photo taken with a decent digital camera, or one minute of a song, or a decent stack of e-mail.