In my opinion, total "market capitalization" makes as much sense as multiplying my number of BTC by the latest market price. If I really want to sell all my BTC today, I will get 5-10% less than simple multiplication. And if there are not enough orders, I will not even sell all of them. More likely, if I exhaust all orders, then there will be people willing to buy the remaining BTC for even lower price than the current lowest (0.0075 EUR/BTC).
Look at this:
https://bitcoin-central.net/order_book"Market price" of 1 BTC does not even exist. There are bids and asks for specific amounts, but nowhere you will find "the price". You mileage will vary depending on how much you want to sell or buy.
"Market" price is an aggregate that we use to evaluate the history of trading and to understand whether it was going up or down. But it's a mere conception, which is very limited. It cannot tell you how much will you gain if you sell or buy X BTC.
The "market cap" is coming to an extreme. If a single person wants to buy all BTC, how much would he pay? You have no idea because there is no order book for all BTC.
Here's my approach how to look at relation between USD and BTC in general. It is not like comparing pounds to kilograms. 1 BTC never equals some amount of USD. There is some approximation of this for low amounts, which works for a OTC trade, but on larger scale it becomes irrelevant. USDs are oranges, BTCs are apples. They are not comparable in abstract. They can only be traded one for another. So the only relevant question that could be asked: how much do I need to pay in oranges for that amount of apples at this point in time? For small amounts you may have approximate "price", but as amount grows, you need to look at the order book.