Bitcoin transactions are sent to the network with an extra fee attached. This fee is paid directly to the miner who adds the transaction to a block. Miner fees are based on the size of the transaction being sent in bytes. Miner fees are not impacted by the amount of bitcoin being spent.
The fee rate is generally measured in Satoshi/byte. A Satoshi is the smallest unit of bitcoin (0.00000001 BTC). For the purposes of this example, let's say a "standard" bitcoin transaction is around 250 bytes in size.
So, a standard transaction with a fee rate of 100 sat/b would be calculated as:
250 byte transaction * 100 satoshis = 25,000 satoshis = 0.00025000 BTC
Simply put, the more unspent outputs that are used to form a transaction, the larger the transaction will be in bytes. By default, Samourai creates larger transactions than average with some of its privacy features (such as STONEWALL). A larger transaction will consequentially incur slightly greater costs in miner fees for a given transaction. You can still configure Samourai to create the smallest possible transactions as needed.
When sending in Samourai, on the Send screen you will see a preview of your estimated fee directly beneath and to the left of the fee slider as you interact with it.
When sending your transaction, on the Send screen you will also see a summary of all fees listed in a popup/dialog box confirming your send for a final time after tapping the green Send button.