I’m lucky in that I have unlimited bandwidth at home, however when I’m out and about there are limits on my data. One thing I realised is that Twitter auto-plays video files unless you change the setting, which I immediately did once I became aware of this.
To update settings to do this on an iPhone, in the Twitter app go to “Me” (left hand margin), then touch settings.
You will see “Video autoplay”. Turn this off, or change to Wifi only.