A lower intensity flame has nothing to do in itself with summer or winter, nor with what gas is being used (butane, methane, propane, etc.).
The flame intensity is regulated by the pressure of the gas as it enters the burner.
So the gas pressure in your lines is lower now than it used to be. If the burner is connected to a bottle or other container of gas, this is an indication that your gas bottle is running on empty and you should start looking for a replacement/refill.
If your burner is connected to the house gas grid with your central heating (for example) and that to the city network, probably the higher demand for gas due to cold weather for heating has led to lower pressure in the pipes and thus a lower intensity flame.
If that's the case, contact the company supplying your gas and ask them about it, maybe they can do something, maybe they're just running at peak capacity and can't increase pressure.