Farlo is spot on.
The power of a foil (windmill, fin, sail) is directly proportional to air density. From 35°C to 5°C, that power increases by roughly 10% (cold air is denser, heavier).
That 10% power surge might appear small but if wind speed and foil's efficiency are kept constant, the power of an 8.6 m2 sail (assuming a luff lenght of 500 cm) used at 35°C is roughly the same as that of a 7.7 m2 sail (assuming a luff lenght of 474 cm) used at 5°C. So, it's no surprise that you feel overloaded by using your 8.6 m2 sail at 5°C whereas you feel nicely overpowered at 35°C.
You can play around with numbers to calculate the power of a foil (i.e., a windmill) here : http://www.engineeringtoolbox.com/wi...er-d_1214.html