I think a few things need to be clarified here.
First, while it's true that there's a 'mandatory' military service of one year for people ( formerly only men ) after finishing secondary school, this military service has very little to do with actual war or combat. They're trained, then sent home, assuming Norway isn't ... invaded. When Norway sends people overseas, it's on an entirely voluntary and highly selective basis. The number of actual Norwegian soldiers in Afghanistan, for instance, numbers in the low hundreds. And the selection process for these individuals is incredibly rigorous.
Also, military service in Norway has always been open to women - just not mandatory.
With that said, every year, around 60 000 Norwegians enter into the age range that may be required to do a year of military service. For various reasons, not all of these are actually called to take the physical exams required. Because this was previously only mandatory for men, the number of men taking their physicals has obviously been higher. In 2012, nearly 7000 women and 17000 men took these tests, and 5500 women and 14500 men were found to be fit for service. ( The requirements for this are not very high; I was found fit for service all those years ago, and there is no conceivable way I could be in the military ).
Of these roughly 20 000, only about 9000 actually do military service each year. It used to be higher, but these days they actually have more volunteers than they can take. That is, if you show up to get your physical exam and you tell them you don't want in, they won't take you.
What this means, though, is the military service that is now 'mandatory' for women, can afford to be highly selective about who gets taken. Statistical differences in biology are irrelevant when you have such a large pool, and the selection is so rigorous.
I hope that clears up some things.
On a personal note, my only problem with the change to the laws in Norway is that they didn't make it voluntary for both men and women instead.