Just some technical background: there is no "default" fee... If you use an (old) wallet that still uses a "default" fee, for your own sake, please ditch it asap.

On average, one block will be found every 10 minutes (the difficulty adjustment tries to make sure the time between blocks is ~10 minutes, so if more hashrate enters the network, the diff will go up to have this number).
A block has a maximum size of 1 Mb.
So, the number of transactions that can end up in a block is limited to 6 Mb/hour (on average). This is why the fee is calculated based on the size of the transaction. Mind you: size has nothing to do with the amount of BTC that is being transferred.

What does influence the transaction size then? Simple: the number of inputs used and the number of outputs generated by the transaction.
A transaction using 1 inputs of 10 BTC to generate one output of 9.999 BTC will be much smaller than a transaction using 100 inputs of 0.001 BTC to generate 2 outputs of 4.9999 BTC.