iTunes says there's no space on iPhone, but there is

Often, when I sync my iPhone 5c, running iOS 9.2.1 to my iTunes account 12.3.2.35 (i.e. both phone and iTunes are up to date), I get a message telling me there is no space to transfer data. But there is, and the message often makes no sense (i.e. it says cannot transfer data, 100 MB required, 600 MB available). The only way I've found to force it to transfer is to deselect a whole lot of data, and to make even more available space, to resync and then put the original data back in again. But I'm wondering if there is an easier way.

I'm also having to back up and restore the phone regularly (every 3 or 4 days now) to keep the data space free - it just seems to bloat up (particularly in documents and settings) and the only way to flush it out is to restore. I've been using a couple of memory managers (CM security and Battery) but these don't seem to help flush out the junk either.

Could these two issues be related?

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