-200 and -300 does not mean a larger airplane. Usually, the later numbers mean a new version.

The 737-400 is bigger than the 737-500.

Before, Boeing started with the -100. My guess is that they wanted to look advanced and have a "newer" version even with the first version. So with the 757 and 767, there was no 757-100 or 767-100. The first version started out with -200.

In recent years, Red China and the Republic of China (Taiwan) have become more influential buyers. They like 8. So Airbus did not make the A360, the next number, but the A380. Boeing did not make the 7E7 but made the 787 upon special request by the Chinese. There was no 747-500 but they jumped to, not -800, but 8 making it the 747-8. The next 737 will not be the 737-1000 and -1100 but the 737-8.