Ethereum is an account based system. It relies on the sender specifying the sequence of transactions to prevent double spending. The balance is associated with an address and when you spend from that address, you can specify what portion of the balance should be spend.

Bitcoin is not an account based system, but structured around transaction outputs. Double spending is prevented by making each transaction output only spendable once. Thusly, transaction outputs have to be spent in full when used. This has advantages and disadvantages: For example this allows funds to be easily mixed from multiple addresses, even if they are controlled by different parties. Also, transactions don't block each other when one is delayed, other than in the account-based model. However, it's unlikely to have the a matching spendable amount to what you're trying to send. Bitcoin therefore allows many inputs and outputs, where inputs reference existing unspent transaction outputs that will be consumed and outputs specify who can spend the money next. In practice the count of inputs and outputs is only limited by the transaction size which above 100,000B becomes non-standard.