Red Ace, keep in mind that 30mm carburettors are what's being mounted Stock on 250cc engines, so it'll definitely be too big for a 125 engine.
An all around improvement (quicker pickup, smoothness, throttle response) could possibly be achieved by mounting a keihin or mikuni, good quality 26mm carburettor if they make them in this size.
A 28mm will probably give you better top end gains, but at low and mid range RPM's it might lose a little.
You have to realize that the size of the carburettor is not what increases performance and/or hp, but carefully choosing the correct size for the cc's and intended application, and the quality of the carb itself.
Generally speaking, a smaller diameter carb will provide more air flow speed, and will perform well at low and mid rpms, buy may not provide enough airflow at very high rpms, thus hurting top end power.
A bigger diameter carb, will enhance high rpm operation, giving you more hp at the top end, but the engine may run worse low and mid rpms.
And even more important than these two, is a properly jetted carb, with the adequate pilot, main and needle jets for the intended use.
Normally the stock (typical) carbs as specified by manufacturers for single cylinder engines are:
125cc --- 24/26mm
150cc --- 28mm
250cc --- 30/32 mm
but these vary wildly, as does jetting, because manufacturers may prefer to have an engine consume less fuel than to have it perform its best, or enhance low rpms, or high rpms, so that's why there is no ''ideal'' carb size for any given engine.