Why such an expensive wedding?

Because the movie is called "Father of the Bride" not "A Family Plans a Sensible Affordable Wedding"

There are so many threads about "why did they do this and this", and complaining about the behavior of Annie, but they don't understand that this movie isn't trying to show people how an affordable, well thought-out and planned wedding might go. The point is that the wedding is a giant metaphor for Annie growing up, and George can't control it, just like he can't control her growing up.

The movie hammers this home pretty early, in this voiceover:

"Then, you stop worrying about her meeting the wrong guy, and you worry about her meeting the right guy. That's the greatest fear of all, because, then you lose her."

This was the tail end of a voiceover about what it's like (for him) to be the father to a little girl. So to him, the impending wedding means that she'll no longer be his little girl.

That's why he initially wants to have the same type of small, backyard party they used to have, where he grills, etc. Yes, the surface reason is that he's sort of cheap, and weddings are expensive, but that's not the real reason. It's so he can retain that semblance of "This is still my little girl." But that doesn't happen, because she isn't that little girl. So the cost/scale of the wedding has to grow beyond his control, because it all represents his daughter doing the same

This continues with the first meeting with the wedding planner. As they're walking there, George says to let him do the talking but later, after they've picked the cake he (initially) objected to because he thought was too expensive, and Franck goes to talk to the two women, George says, in voiceover:

"With one swift move, I had been cut out of the deal"

Later, we see more and more decisions taken out of his hands (The tulip border and the swans, for example). Meanwhile, this same loss of control happens outside of the wedding planning as well. When he tries to get her to put on her jacket because it's cold, she insists she's fine—but she puts it on when Brian suggests it five seconds later.

George then wrestles with all of these feelings about Annie in a voiceover at the church during the ceremony.

"But she's not a woman. She's just a kid. And she's leaving us. I realized at that moment that I was never going to come home again and see Annie at the top of the stairs. Never going to see her again at our breakfast table in her nightgown and socks. I suddenly realized what was happening. Annie was all grown up and was leaving us, and something inside began to hurt."

Even after the ceremony, this theme continues. There are too many cars to park, and George has to handle that and as a result, he misses seeing Annie at the reception. It's all one giant metaphor.