The angle of the cylinders was reduced to reduce the propensity of steeply angled cylinders to lift and lower the driven wheel with each thrust. The development of 'Rocket', the 'Northumbrian' class were built with the cylinders at the lower angle.
The Science museum produced a technical booklet about 'Rocket' some years ago which went into forensic detail over the remains as they are currently and showed that in its' short working life the loco underwent significant changes. Technological development of the steam locomotive was moving at such a pace that it was outmoded almost immediately upon entering service.
Regards,
Phil