The fee size is relative to the amount of competition to get transactions into the next block. Because the block size has been capped at 1MB, not every transact

The fee size is relative to the amount of competition to get transactions into the next block. Because the block size has been capped at 1MB, not every transaction in the preceding 10 minutes will fit. So miners sort by the fees attached to the transactions when selecting them. (I think the block size is supposed to be increased to 2MB shortly, if it hasn't already.)