How to discover which tab is taking up the most memory in a spreadsheet?

I do not know of any tool that shows memory usage by tab - you may just have to make a copy and delete the tabs one by one to see what has the most improvement.

do a binary chop - delete half the sheets, find which half had the biggest load, repeat...
25  --> 12  --> 6  --> 3 --> 1
Should have found it after 5 deletions, worst case.

If you are using the latest format for your workbook, i.e. xlsx, then I believe that you can open the file in a tool like Winzip or 7-zip (you can, but shouldn't have to, rename the file to have a .zip extension) and browse into the /xl/worksheets folder in the archive and see the relative sizes of the tabs.

Hi, at work we have something called PKZIP.I have zipped it and looked at it in Windows explorer and I do not see the different names of the tabs, just the name of the file with a zip extension. How do I browse within the xl/worksheets folder in the archive?

and in terms of memory, are we talking file size on disk, or RAM occupancy - these could be quite different...

I get the message "Excel ran out of resources while attempting to calculate one or more formulas. As a result, these formulas cannot be evaluated" Here's the file. Please help!
03-Mortgages-EMEA-Loan-Exposure-Live.xls

Hi. You probably need to open a new question for this. Had a quick look into this and all I can say for now is that the workbook seems to require ~220MB of working memory upon opening and this increases to over 500MB when you trigger a calculatіon of the workbook formulae. So consider how much RAM you have on your machine and what else you have open (thus using RAM) when you use the workbook. Also note that there is a linked workbook "Loan_Reconciliation_28022015.xlsx" that will be using additional resources in the background.