The average vehicle weight of a 710 is 36 tonnes? Compared with the 34.5 for a 700, there's not a big difference. Considering the 710s are only 4-car so a greater proportion of the unit is the heavy cab, transformer etc. that seems about right.
Cabs add about 1.5 to 2 tonnes each.
Class 710 (5 car) dual voltage is 36.4 t /car average.
5car 707 is 32.4 t /car average with transformer and pantograph removed and less traction equipment (2 fewer traction motors, the remaining motors being smaller and lighter, 1 fewer set of traction electronics).
Class 700 (12 car) is 34.2 t /car average
Class 700 (8 car) is 34.8 t /car average
Bombardier and Siemens have very different equipment location strategies so you can't just compare the lightest Siemens car with virtually no equipment fitted to the lightest Bombardier one with some equipment.
The Bombardier 710 driving cars are also about a metre longer than the Desiro City ones.
TfL also require greater equipment redundancy for LO.