I've asked this question before and consistently failed to get a clear answer - why is there any deviation between index fund weighting and market cap?

To some extent, I'm sure the definition of a "public" company comes into play. Not all stocks are traded in all exchanges, so you could include stocks only listed on one exchange.

Then there's the practice of many index funds picking the top N stocks by market cap. This seems like a backwards practice to me. The small cap stocks should be limited in weight by... their small market cap.

Then there are other hairy factors. Even out of the stocks in an index, it seems that weight does not correspond to capitalization. The reason seems to be some historical drivel. While I can understand that is the way it is, I fail to understand why it should be that way.

Why should an equities index fund be anything other than public companies proportional to their size? If people prefer large cap or small cap, then those variations should be offered as special boutique products. But it seems that we have it backwards, where the default offering is based on arbitrary non-proportional weights, and with a cutoff restricting it to large cap.