system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why isn't the US dollar the most valuable currency?

So as my fiancé and I are planning our honeymoon trip to Europe, the topic of conversion rates came up and she asked me why the Euro and the British Pound are worth more than the dollar and i didnt really have a good answer. Now, the Euro I kinda get, because its a multi-national currency backed by the entire EU rather than one country, but the British Pound is where I get stuck.

So reddit: How can a small country with a GDP that is ~1/5th the US have a currency that is more valuable (at the time of this posting $1 = £.64) than the dollar? 

EDIT: a common thing I'm reading is the confusion between value/purchasing power and exchange rate, and clearing that up helps quite a bit. Thanks everyone!
gpt
Think of it like this. You have 10 bananas, your fiancé has 10 apples. They are different, but tastes aside, you agree that that they are equal in terms of fruit "value". Now you want to trade fruit, but not whole fruit. You want a complicated system where you can trade a part of a fruit for basically an IOU-note so the significant other can get fruit back when he/she feels like eating some.

You decide you will divide each banana into 10 pieces. You now have 100 bananaslices. Your fiancé just splits her apples in 4, and has 40 appleslices. You then represent each slice with an IOU-note you can trade back and forth and cash in for the corresponding fruit type when you feel like it. This would make the exchange rate 2.5 bananaslice notes for 1 appleslice note, or if going the other way 0.4 appleslice notes for 1 bananaslice note.

This doesn't change the fact that your resp. wealth is equal. It's just that 1 unit (one note) of your currencies represent different amounts of your total wealth.

What Alikont talked about is more about what can make those exchange rates change over time. 

You may both get a craving for apples and start to dislike bananas, making the banana notes more sought after. This would make you both more willing to give more than the previous exchange rate to get the other to agree to trade.

You might decide to cut each bananaslice in two and in a dictatorial manner say that each bananaslice note now represents one of the new smaller slices. This means you just got a bunch of banana notes richer, while any banana notes in your spouces possession suddenly lose half it's worth. She'd likely be pissed about this, and start requiring twice as much bananaslice notes for each appleslice note. Voila, the exchange rate changed without any actual change in total fruit "value" in play.

Hope that helped. :)