RE: TRX 2.5R vs. HPI S-25?
The 2.5R is a newly renovated TRX 2.5 (which came in the Tmaxx 2.5). It is speculated to have more power than the TRX 2.5. The TRX 2.5 was a problematic engine for some. There were some quality issues with a few batches of this engine. There aren't all bad, only a small percentage left the factory less than perfect. If you got a good one, that engine really screams. Traxxas has listened to people's complaints about it and made an effort to improve it, hense the TRX 2.5R.
Like Thermal said, the reason the Maxx and Revo have a small block compared to the Savage, is weight. They weigh less, so they don't need as large an engine to maintain a good power/weight ratio.
The differences between a small block and big block are; rpm, HP/torque, and size/weight. Small blocks generally produce less power and torque than big blocks, but small blocks also produce higher rpms than big blocks.