There is evidence that big weddings correlate to a lower divorce rate so it might not be pure waste. If nothing else a big wedding creates a clear separation of married vs unmarried life. Much like a difficult cumming of age ceremony may influence long term behavior.
Then again it's hard to separate the mindsets of people that get married at a justice of the piece vs inviting 100+ people to the big event. I suspect cost is less of a factor than the amount of effort spent planning the event as thinking about a wedding probably promotes it's importance more than it's cost or the day it's self. aka deciding which table cloth to use is not important, but thinking the wedding is important enough that the table cloth choice is an important decision is probably meaningful.
Does this evidence really show that expensive weddings decrease divorces, or could it be that expensive weddings signal marriages that are less likely to end in divorce?
Consider:
Parent of the Bride or Groom: "I really like [whoever] and think they are great for you; let me pay for your fancy wedding."
Parent of the Bride or Groom: "I really think that [whoever] is sketchy and is wrong for you; this marriage will not last, pay for it yourself."
Or potentially even:
"We are two young successful people in love, lets get married."
"We are two young impoverished people in love. Let's get married, but since money is a source of constant stress and anxiety for us, let's keep it cheap."
Either way you slice it, divorce rates are high so if a divorce is an idea that concerns you, you should probably not get married.
Note I said big not expensive weddings as large weddings need not be expensive. It could simply be that having a large extended family and or lot's of friends close by is the important factor.
As to not getting married, married men live significantly longer. Again that may speak more to the type of people who get married than the actual value of marriage. However, anecdotally having someone else push you to go to the doctor and follow there advice probably has significant value. Still there is a lot of evidence that not getting divorced has significant value now many people think that relates to who your spouse is but the number of strong / successful arranged marriages suggests having the right mindset is vary important.
Either big or expensive, I don't think the direction of causation there is clear without more information. The number of attending family members could easily signal the number of family members who support the wedding or think that the marriage will last.
(The size of a wedding is also closely related to expense. Large venues are more expensive, and catering/booze is more expensive when you have more people.)
"There is evidence that big weddings decrease the changes of divorce"
Nope, in our case, being a bit older, stable higher income careers, somewhat older lifestyle meant two unrelated things 1) we could spend money that lovesick teenagers could never hope to afford 2) Being older / stable / whatever you want to call it, means the marriage can't suffer growing pains when we become older / stable / whatever so divorce is much less likely. I think it a fair assumption that people change a lot more from 18-28 than from 28-38 or 38-48. From looking at my parents I don't think they changed a lot from 38-68, although I don't wanna know whatever they did as hippies before they got to 38.
I think these two conditions internally correlate much better than they correlate with each other, although anecdotally I suppose it randomly happens to be more than a minor coincidence.
Then again it's hard to separate the mindsets of people that get married at a justice of the piece vs inviting 100+ people to the big event. I suspect cost is less of a factor than the amount of effort spent planning the event as thinking about a wedding probably promotes it's importance more than it's cost or the day it's self. aka deciding which table cloth to use is not important, but thinking the wedding is important enough that the table cloth choice is an important decision is probably meaningful.