More space in London Underground trains

Space is greatly lacking in the London underground trains. The space is constrained by the size of the tunnels. The size of the tunnels could be enlarged but this would be prohibitively expensive. So another solution would be to modify the trains. I remember being very disappointed by the most recent designs of the trains because they had become smaller. I can only think that this was due to a grave error. For the Northern line at least, the space problem has been exacerbated. You only have to look at the thickness of the walls and behind the seats. All that width is wasted for what reason? I have been on many underground and overground trains abroad and I find it incredible that the most recent designs require so much width for the walls. In contrast, the underground trains in Tokyo are incredulously thin. Don't squander so much space for the walls.

Another way to increase the space is to consider having the floor lower. But this may mean lowering the platforms and perhaps making the wheels smaller. The size of the wheels could be unaltered and cleverly housed underneath the seats. Although the heat from the wheels could be felt more by the people sitting down.

More thought is required but I do feel that more space can be claimed by the passengers.