First of all note that I do not consider myself an expert on load time optimisations, and you may be able to find better advice elsewhere using a web search.

That put aside, here are some factors that may contribute to this:

1. Some of it has to do with my web hosting, currently HostGator. While not perfect, I am quite happy with them, and can recommend them for shared hosting.

2. It is possible the site does not receive a lot of traffic for a variety of reasons (not sure).

3. I enabled server-side gzip/deflate/etc. compression.

4. XHTML/HTML, CSS, and JavaScript are all minimised.

5. I use common external stylesheets, script files, and images instead of embedding them on every page.

6. The website is mostly static and served from the server's hard disk directly without potentially costly server-side scripting.

7. I avoid loading unnecessary JavaScript codes and other resources from other sites (with some possible exceptions.).

8. PNG images are compressed using OptiPNG ; SVG images are used as well.

   Moreover, I recently started to increasingly make use of WebP.

9. I sometimes remove various embellishments of the site, that while seemed a good idea or were one at a time, are no longer needed.

10. The navigation menu is partly populated after the page load by using an XMLHttpRequest (or so-called "AJAX") call to a static JSON file.

Some factors may play against the site's responsiveness (e.g: use of XHTML instead of HTML, or use of valid markup).

Also see:

1. Tweet about average web page size

2. Wired feature about the average web page size