A transaction can have many inputs and many outputs. These are all Bitcoin addresses.

Sally and Bob each have 1 Bitcoin. Sally received her bitcoin to a single address. If Sally wants to send that 1 Bitcoin to Charlie, that transaction would have 1 input (the address she received it on) and one output (the address Charlie wants to receive it to). Bob has 1 Bitcoin total, but his 1 Bitcoin came from 4 x 0.25 transactions. Bob didn't want to reuse addresses, so for him to send 1 Bitcoin to Charlie he has a transaction with 4 inputs (each address which contains 0.25 BTC) and 1 output. Thus, Bob's transaction is much bigger than Alice's, despite the same amount of Bitcoin being sent.

Thanks, that makes a lot of sense now.