Why was mass the factor for the London Eye and not volume?

Question: Why was mass the factor for the London Eye and not volume?
- Keywords:
  London Eye,
  mass,
  volume
Asked by to Thomas, Saiful, Piers, Olivia, Meggi, Laurence on 13 Jan 2017. This question was also asked by .
- Saiful Islam answered on 13 Jan 2017:
  
  In looking at different fuel types, we wanted to show trends in the gravimetric energy density (based on mass) – using the London Eye rotations has a nice visible comparison. I agree with you – if we had more time (and resources) we could have included volume as a factor as well. In any case, we wanted to show the general differences – which I think came across. On a personal level, I did like our time lapse filming of the London Eye.

Question: Why was mass the factor for the London Eye and not volume?