I have had this happen where you start out with your chain foundation with an afghan or scarf and it is narrow at the bottom and wider at the top. What you can do it start off with a very loose chain. If that does not work enough, you can use a larger crochet hook. What we are trying to accomplish is both ends being the same or very close in size. Sometimes the chain will be crocheted tighter than the rest of the project which makes the starting end narrow and the finishing end wider.
I have also found that the foundation crochet stitches also known as chainless crochet, are a great way to make sure both ends are the same. With the foundation stitches, you are creating the chain and stitch at the same time. I will use this method instead of the chain.
Chainless Single Crochet Foundation
Chainless Half Double Crochet Foundation
Chainless Double Crochet Foundation