Actually I don't think that is a Transformers concept solely found in Beast Wars/Machines. Even though 'sparks' weren't called by that name until Beast Wars, presumably they have always existed. Do Galvatron, Cyclonus and Scourge still have the same sparks as Megatron, Bombshell and Thundercracker? Does Rodimus Prime have the same spark as Hot Rod? If so, their sparks' behaviour does seem to be clearly affected by their physical shells (or yes, by Unicron and the Matrix). In the Marvel comics, after G1 Ratchet turned Goldbug into Pretender Bumblebee, Bumblebee became more confident, a consequence of other Autobots looking to him for leadership on the battlefield for the first time (as stated in 'Yesterday's Heroes!') This suggests that Transformers' personalities are affected by their physical shell... so to speak. And even in these comics, in which the Decepticons were not speciesist or sizeist about themselves (they took orders from Ratbat as readily as from Shockwave, Scorponok or Megatron), I still find it unlikely that Megatron would be the same Megatron if he were in Ravage's diminutive quadrupedal body. (And I don't even want to get into the Straxus's-mind-in-Megatron's-clone thing, because that whole storyline does my head in as readily as it did his.)
In Beast Machines, Waspinator, Silverbolt and Rhinox became polar opposites in the bodies of Thrust, Jetstorm and Tankor. This could be a result of Megatron's 'shell programs', but both Thrust and Tankor demonstrate at times that they are in fact still the same 'person', they've just chosen to behave completely differently in their new bodies. Also, like G1 Bumblebee, Transformers are influenced both by their impressions of themselves and the impressions of others. Beast Machines Megatron was the same 'person' when he was a bestial dragon, diagnostic drone, an 'Optimal' Optimus and a BFHead. But he behaved and saw himself differently in all these forms (with disgust, embarrassment, kinky satisfaction and bwahahahaha-ness! Respectively.) And the Maximals reacted to Megatron differently depending on his form too.
Transformers history is full of examples where the same spark in a different body behaves differently. This could be for technological reasons or it could merely be the psychological result of feeling more confident or less so depending on one's current physical form.