France was really playing against the odds here
their alliance had about 100k troops and we had about 250k (and this is public knowledge inside the game, no hidden strengths).
Yet this isn't a necessarily a foregone conclusion. Me and England won twice when it was 2:1 the other way round.
Indeed I lost quite a lot myself, about 50k in the end from not paying careful attention and many a siege of mine was lifted by small armies when I tried to chase the large armies.
What really got France was that they had a ticking clock against them while they did not control London (their war goal as attacker) and that isn't easy even in good circumstances.
They also obviously had to fight on two fronts (three when Austria joined later, but that barely mattered at that time). And since France (the region) was already very fragmented, we (the English alliance) had a lot of space to manoeuvre. The French couldn't bottle us up and defeat us in detail.