Because you pay fees according to the data you brodcast to the network ? So it's units, sats per vb

Check out this thread. It explains the difference between bytes and vbytes: https://bitcoin.stackexchange.com/questions/89385/is-there-a-difference-between-bytes-and-virtual-bytes-vbytes#89418

Some transactions take more space than others, depending on a few factors like if its a native segwit transaction, how many UTXOs had to be used, how many outputs there are. You could have a block that only fits 100 transactions, and another that fits 3000, just due to the size of the transactions; ...

The larger the transaction, the more it costs to get included in the block chain. Would you pay 10k sats for a monkey JPEG? Imagine when bitcoin is at a million bucks.