In 2018, a mobile webpage should be no larger than 500 KB. This recommendation is based on a widely cited marketing report (web|pdf) from Google "Find Out How You Stack Up to New Industry Benchmarks for Mobile Page Speed" (2017, Daniel An), which marks 500 KB as a "best practice".
It is unclear from the article exactly how this number was derived, but I believe the line of reasoning is as follows:
- Goal: Minimize website bounce rate.
- Model risk of bounce as a function of webpage load time, itself a function of webpage size.
- Pick a best-practice webpage size that most greatly minimizes the risk of bounce.
Specifically, the article reports the following benchmark results:
- 1s to 3s the probability of bounce increases 32%
- 1s to 5s the probability of bounce increases 90%
- 1s to 6s the probability of bounce increases 106%
- 1s to 10s the probability of bounce increases 123%
I believe the article is inferring that the greatest gains lie in the range under three seconds, which in turn is the load time used to derive the page-size recommendation of 500 KB.