One of the good things about LTE is that especially in the radio network, it represents a fresh start so a lot of "optionality" that tends to bloat a specification over time is avoided in the general baseline. To see what I mean, let's compare the Radio Resource Control (RRC) specification of UMTS with LTE:
The original Release 99 UMTS RRC specification (3GPP TS 25.331 v 3.1.0) is about 865 pages long. In Release 8, the same specification document (v 8.1.0) now contains 1435 pages.
The RRC specification for LTE can be found in 3GPP TS 36.331. The December 2009 version (v.8.8.0) contains 208 pages. That's only a fraction of even only the R99 UMTS specification.
I am a bit surprised the volume difference is this big. I compared chapter headings to see if there are things that the UMTS RRC spec contains which are not in the LTE RRC. However, it all looks pretty similar.