I made a transaction of 1.22242 mBTC with the electrum client and, I don't know why, it looks like it transferred all the remaining BTC from my account to myself which made a 13063 bytes transaction and costs me 8.43769 mBTC... Do you know what happened?
This is a process known as consolidation - If your wallet has several small inputs, many wallets will often try to combine them into a single change input during regular transactions, especially ones made with a low sat/byte feerate. This allows you to spend the coins in a smaller transaction during possibly high traffic times, when the feerate may be much higher.
These days, many wallets only perform such consolidation if there are many outputs to a single address, as combining outputs from multiple addresses unnecessarily can lead to a loss of privacy. Unfortunately for you, it seems like in your case all the outputs do belong to a single address.
In the case of electrum, this doesn't appear to be a configurable option, although you could notice it is happening prior to submitting the transaction by using the "Preview" option - if you notice that the total fee or size or number of inputs consumed it too large, you can force electrum to build a transaction using only specific inputs by selecting them in your transaction history, then right clicking and selecting the spend from option.
The problem is the transaction fee doesn't depend on the values of inputs, but the number of inputs.