PBRT is big because it is written using literate programming and therefore includes (almost[1]) the entire source code for the ray tracer in print, which arguably makes it easier to follow than other books on the topic.

The PBRT ray tracer is also a production quality ray tracer and uses many advanced techniques you won't find hobby ray tracers, such as differential geometry, which naturally increases it's size.

[1] Some utility classes are left out.