Steps to reproduce:
1. Download the projects for 5.6 and 2017 versions
2. Build each of them and deploy to an iOS device (developer mode with "Autoconnect Profiler" should be enabled)
3. In Unity under "Profiler" window, select device the build is running on under "Active Profiler"
4. Check the detailed memory usage

Results:
5.6 - 23.1MB of ram used, Scene Memory takes up 8.8MB, out of which Texture2D - 8.8MB
2017.3 - 35.7MB of ram used, Scene Memory takes up 11.6MB, out of which Texture2D - 11.1MB

Reproduced with: Unity 2017.1.2p1, 2017.2.0p1, 2017.3.0b9

Reproduced on: iPad 3 Mini, iOS 10.3.2

Note: UnityWebRequest uses the same amount of memory in 5.6 and 2017 versions.

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Issue will not be fixed: Since this issue does not reproduce using UnityWebRequest, it won't be fixed, please use UnityWebRequest.