It isn't. Actual new regulatory content is only a small part of what's in the Federal Register when someone breathlessly tells you "X-hundred pages of regulations came out!"
The federal government has to release a first draft of regulations it's considering and let the public comment on it. It then has to take those comments, essentially one by one (depending on how important or of wide interest a proposed rule is, there may be hundreds of even thousands of public comments), and decide with respect to each whether it warrants making a change to the regulation as proposed or not. When the final rule gets published, it will contain dozens and dozens of pages just walking through the comments members of the public submitted and explaining what changes the relevant agency has or hasn't made based on each one (or each set of thematically linked comments).
The reason a final rule is long is because the government has to engage with the public on the input it provides, explain its rationales for any regulatory change it's making, provide a number of mandatory economic and financial impact analyses, etc.
A 100-page final rule may have a handful of pages of actual additions or revisions to the CFR. The rule itself is long because it's a super annotated narrative document that provides everything anyone could need to know about what it is, where it comes from, why it contains what it does, how it's applied, what interested members of the public thought about it, what the agency did in response to each of those thoughts and why, and what the impact will be.