Volume, i.e. number of second level domain names, is one way to measure the size of a TLD. It may not be the best one, but it is the most reliable measure we have to date.

We could try to measure the presence on search engines – google visibility could be one proxy – or the estimated traffic on domains – alexa traffic for instance.

Looking at the size, we see the ranking of the largest domains is quite stable over the months – at least for the 20 most popular ones.
.xyz is first, with a comfortable lead..
.网址, meaning .site in chinese, is second since January.
.science came in the top 3 in May, taking over on .club.
A newcomer in May was .party, who jumped directly on rank 5.
The newcomer in June is .work, arriving in the top 20 at the 18th position.