Hey Greg,
I'm sort of answering an earlier post too...
Lightroom is a parametric image editor - nothing you do in LR affects the Camera raw files (or the original tiff if you are working from a scan) and you can always return to the original file state. Photoshop is a pixel based image editor and once you are working in PS (ACR portion of PS is a parametric editor like LR) you are fixing the pixels in place.
Save a color tiff file as a BW in PS and you can't go back to color once it's saved, in LR you can go back.
LR saves instructions sets (parametric editing) and leaves the original files unchanged, PS acts directly on the underlying files and alters the actual pixels once saved (pixel based editing). PS has some new parametric features (cropping comes to mind) and the ACR portion is non destructive too. You are correct that adjustment layers do not build file size, but retouching layers (cloning and spotting etc) will build file size quickly.
LR has improved quite a bit but still can't compete for deep image editing beyond simple spotting and standard image adjustments, but for the basics its hard to beat, especially with the non-destructive workflow.
There is a really good full explanation here by my friends Richard Anderson and Peter Krogh on dpbestflow-
http://dpbestflow.org/links/36 - well worth the read.
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