its true that power is measured in watts not in volts but as u said that ur bike has old wiring which u havent changed so its absolutely possible that it might have become weaker with the passage of time ...
a 6 volt 25 Watt bulb will consume 4.166 Ampere current
where as a 12 volt 25 watt bulb will consume around 2.11 ampere current
old wiring cant support high current but yes they can support high voltage so its a good conversion u have done and u will experience a brighter light
only draw back can be the quality of winding they have done , but rest it will give u improved results