Put yourself in the other mum's situation, and try to envisage all the reasons why they might not have invited everyone - most of which are entirly innocent, and where they'd have asked more if circumstances permitted.
Obvious one is if they are holding it at a venue where they have limited numbers, or where they charge per head (as some places are can be £8 or so a head, then asking 37, plus any non-classmate friends could prove rather expensive).
I've just been organising ds's 4th birthday party, and with a limit of 20 we just can't ask everyone even if we'd like to.
So I've had to risk upsetting some, by limiting it to kids between 3 and 6, and putting in a note explaining that we couldn't include younger/older brothers/sisters. Last year we had a bit of a nightmare with excess numbers when I had responses from a couple of parents saying that x and y would be coming, when I'd only invited the one who was actually at ds's nursery.
Even then, once I've allowed for neighbours, and non-nursery friends, there are only 8 places left between the 13 in his pre-school group. Now do I ask the ones who have been there longest, and he therefore knows best, but who will be leaving to go to school this autumn so he'll probably not see them again afterwards? Or leave those out and ask the ones who will still be there next year, so they hopefully reciprocate and ask him to their parties too over the next year? Or just the boys? Whatever I do, I'll potentially upset someone.