Its all to do with the chemistry inside- more electrolyte layers inside and so on. The weight fluctuates a bit but by very little. You can pretty comfortably say that Sub-C cells all weigh pretty much the same, or at least the variations arnt too big.

Its the chemistry that makes the difference. Traditionally Ni-mhs have a larger capacity that Ni-cds, and they dont suffer from memory effect so much. However Nicads can be discharged faster (i.e. you can pull higher currents from them) than Ni-mh. Thats quite a black and white way of looking at it, and thanks to advances in technology we have got to the point that some Nimhs cells are beginiing to outperform similar Nicd cells in terms of discharge rate, thus pushing Nimh into the lead so to speak :-) A case in point are the recently launched 2600SCU nimh cells, which are said to outperform the RC2400 cells (nicd), as well as having the obvious advantage of higher capacity.

As for the Nimh not getting warm, it depends on what youre doing with them. If you start pulling more than about 45A they start to get pretty warm. This is the point where having them spaced pays dividends. General word of mouth rates them at 45A continuous, 60A for a short time. The RC2600 cells, by the same token, are said to be good for 60A continous and 100A for a short time. Its all just a question of looking at whats best for your application.