Fills up how? With what? Pictures? Files?

Most likely, some app is caching data and not cleaning up after itself.

Malware from porn sites?

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The secret of a long life is you try not to shorten it.
-Nobody

A phone model and OS your running might be helpful for diagnosing the issue.

My first Android phone had issues with the email app cache not being really cleared even when I deleted things off of it. Known glitch that was eventually fixed on the next model. Solution at first was to do a reset but it would fill up again. Eventually switched to a different 3rd party email app that wasn't the built in one that comes on the phone (this was before I used gmail and was using a hotmail account). Phone was an HTC Droid Incredible, 1st gen. .

Delete on your device would give us a starting point in troubleshooting.

LG and android

Do not understand