The thing is, if miniaturisation of circuits and other machinery is so widespread and corridors are so numerous , it should be a simple matter to position the mini-machinery adjacent to the existing corridor structure. Having to build EXTRA service crawlways in order to access a comparatively tiny amount of machinery just doesn't make good design sense (and that's before you factor in oddities like the vertical JT's low ceiling height).
Of course, the crawlway-style Jefferies Tubes were never a part of the original ship design, but introduced and expanded upon in the 5th & 6th seasons of the show. The fact that they were never thought of from the beginning does help explain why they fit so poorly into the layout of the ship as were initially led to understand it.
Miniaturisation of technology? I tend to agree - probably advances in food synthesizer and automated manufacturer techniques allowed former bulk cargo spaces to be replaced with expanded with sciences and other faculties.
Incidentally, I took the time to revisit Ed Whitefire's original deck plans for the E-D and was surprised to see how closely it fitted my posturings upthread for a mostly non-living quarters layout for the saucer
http://www.cygnus-x1.net/links/lcars/ed-whitefire-enterprise-ncc-1701d.php
Compare that to the
"official" plans, where pretty much every spare inch of space it filled with crew quarters (most of them twin berths) and I know which version feels more like a hotel and which version feels like a functional deep-space starship.