is iCloud burning data through my Personal Hotspot?

Hello

I am burning a lot of data on my mobile/cellphone plan whenever I connect via Personal Hotspot

Mainly I am sending emails from MacBookPro via iPhone when out of Wifi range.

This is burning a lot of data which my carrier Vodafone UK charges for.

I have a monthly plan with 2Gb allowance ... that I thought was more than I needed

(although I note from some threads that the cognoscenti have 10G+ plans)

I did a test sending myself an email with a 600k attachment and then disconnecting hotspot.

That 'cost' 7.5Mb data (Settings > Mobile Data)

Another time I sent a couple of emails and it managed to burn 400Mb

MBP desktop is set to backup to iCloud ... could that be the culprit?

Is there a way to stop it doing that when on cellular?

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MacBook Pro MacOS 10.13.3

iPhone 6S IOS 11.2.5