It appears that inexplicable bloat in Excel workbooks is a reasonably common issue, so my experiences with this phenomenon may be useful to others
A shared workbook had blown out in size considerably for no reason that I could see (ie from 4M to 33M)
I had already gone through and removed as much unnecessary formatting as I could (it's a shared resource) but it made no difference
Googling didn't turn up anything particularly useful (the main fix seems to be removing unnecessary formatting, which I'd already done), but it did remind me that office 2007 files are zipped xml bundles.
When I opened the (zip) file, I found 2 worksheets were considerably larger than the others (one was 180M and the other was 89M). AHA! says I
I went back into Excel and MOVED the sheets out, saved, then checked the size, and...no change
I went back into the bundle and found that the bloat had moved to the next sheets (WTF?), so instead of sheet26 being 180M, now sheet25 was 180M.
I made a fresh copy and this time I COPIED the offending sheets out, then went back into the bundle and DELETED the offenders, saved, then checked the size.
This time the file had come back to a more reasonable 4M, so I re-opened it with the intention of copying the deleted sheets back in.
On opening the file Excel threw up a 'this file is corrupted, do you want to repair' message to which I answered yes. After letting it do it's stuff I saved and went to copy the deleted sheets in only to discover that Excel had automagically reinstated them!!
I've done as much testing as I can (the workbook contains data only, no additional functionality) and as far as I can see no data was lost to the exercise, and the aforementioned bloat is now gone (woohoo...)
I can't vouchsafe this method, however it seems to have worked in this instance. YMMV, so if you're going to use it I would suggest you make a copy of your file(s) first, then confirm the results yourself before you replace your originals.