If after working a few inches of your project you notice that the piece is getting wider and you didn't add any stitches, there's a chance your foundation edge may be too tight. The chain stitch is easy to make narrower than the other, more structured stitches, so accidentally making the foundation chain too tight is quite common.
To avoid this issue, try to keep your chain stitches loose as you work the foundation chain. To keep your stitches loose, you can either consciously make loose stitches by keeping your tension looser than you would for regular stitches or you can use a hook that's one size larger than the one the pattern calls for. The resulting chain may look a little messy, but some of the slack will be taken up when you work the following row of stitches.
Keep in mind that your foundation chain should look like a big fat worm!
