If it was strictly ceremonial, I would expect the sizes of the stones used to build the site would have been overall of a smaller scale.
I agree with you, but we have to remember we're talking about a time when something ceremonial couldn't be termed "just" ceremonial. Ceremonial was no small thing. It was everything. This is a time pre-science, when everything, your food, well-being, victory in war, fertility, etc. was in the lap of the gods. If you don't build it big and lasting for the gods, you don't build it big and lasting at all.
Churches, in the middle ages, remained the largest buildings in any village (bar perhaps the castle). Religion has always got the biggest and most lasting buildings. Not to digress and get political, but don't we do the same nowadays? We "moderns" worship money and wealth above all things. Nowadays, big business gets the biggest, flashest, tallest most lasting buildings.
But I totally agree with you that the solstice alignment thing is CLEARLY not accidental. I just don't believe that "the huge calendar" theory is quite true. This was a church first and foremost. It's alignment is no more significant than the fact churches are built on an East West alignment. Future cultures would be wrong to assume the Christian faith was tied to the rising and setting of the sun. It is secondary symbolism.