By default, Unity scales textures to the nearest power of 2 on each axis: 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192 and so on.
For example, an image that starts at 31x4000 pixels might be imported at 32x4096. That’s a weird size, but it gets the point across.
You can change this by editing your import settings for each image. Unity tries to pick default settings that are generally good for many different games, but it’s impossible for them to cover every use case. Sometimes it’s necessary to tweak the settings.