Opening 118,8kB xlsx file and saving it without modificating will increase its size to 2,1MB. Saving the same file to ods produces 108kB file. Ubuntu 14.04.1 with LO 4.3.5.2 installed from LO 4.3 PPA. Previous version 4.3.4.x didn't have this bug (update applied 23/12/2014).
The file in question is shared with colleagues (MSO 2010 of course) so it has to be in xlsx. I use 4.3 from PPA instead of stock 4.2 in my Ubuntu due to fixed bug I reported (https://bugs.freedesktop.org/show_bug.cgi?id=81080) and also one bug I didn't have time to report but is fixed in 4.3 too.
I just found I modified one other xlsx file after the last update and that file is not affected. When I looked at whats different I found that some rows and columns have background colour and the colouring goes to the end of those rows/columns (way beyond the data). I copied one such sheet into a new file, saved it (768kB) and then deleted content of cells after/bellow data and erased formating too. After saving the size decreased to 53kB! So the culprit may lie in bad formating practice, however the previous version 4.3.4 was fine about this and also ods version is handled properly in the new version 4.3.5.2 too. So I think it's still a bug (or enforcement from LO to teach us proper formating practices?).
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