I never payed attention to updates before, wasn't necessary. Until Windows 10 came long and I'm a little cautious. Because that one update might break something.
"Also, since this is a cumulative update, it includes all previously released fixes and improvements from past cumulative updates, so in case you missed one, you’ll catch up with everything with KB3185614."
That'll be why; it's basically a bundle of all updates to that point.
Bug in windows update API, says the same thing on mine but C:\ is not at 180 GB more used space than it was before march (and neither are any of the other drives).
My understanding of it, the size reported is the total download size of files for every version of windows (home/pro/enterprise/country or region specific variants/etc), in every language, in x64 and x86, and possibly some other duplicates, so that is many times the actual size that would be downloaded on a single PC.