Why do you argue with absolute numbers? That would only make sense if the EU would be a super state, where all the money and economy is centralised? It is not. The EU is an Union, an umbrella organisation/federation for 27 countries which is supposed to create a place to work and trade with each other.
The difference in the economical power scale and consequences can actually be summed up rather easily. As long as the UK won´t be a part of the single market, 27 individual nations will have a harder time to trade with a single country. That country on the other hand will have trouble to trade with all of the 27 nations. It loses easy trading access to a whole region. The damage for the UK will be concentrated completely on them, while it will be split amongst all of the EU members. They would have a way easier time to deal with that than the UK.
Let´s take my country for example. We are the most affected by the Brexit as we are their most important business partner in Europe. In theory from an economical standpoint we have the biggest reason to allow the UK to stay inside the single market. The problem here is that we also have the biggest political reason for not letting them off easily here and offer them a good trade deal. We want a stable EU, we need a stable EU the most out of all EU members. While we are the biggest contributor to the Union, we also profit the most from it as we are not only the biggst economy but also in difference to the UK an economy clearly geared towards export and the majority of our wealth comes from that.
The government will try everything they can to prevent a chain reaction of other EU members leaving from happening. That is why our political leader, our finance minister and our main guy in the EU made some pretty serious statements today.
The upcoming negotiations will be pretty damn tough for the UK, especially as the Brexit has created a pretty damning split of the country. The possiblity of Scotland leaving is very real and as they clearly showed their will to be part of the EU, I don´t see any reason for them to be not welcomed back. Without the Scots the UK will lose even more leverage in these negotiations and as the UK are in for years of economical uncertainty, I can see a pretty big part of one of their key sectors (finances) moving away aswell into the heart of the EU and their biggest nation. The prospect of Frankfurt succeeding London as financial capital of the EU might very well play a role in Germany´s stance aswell.