Check that your feed dogs are up and clean. Check to make sure the stitch length is set for bigger stitches. Check to make sure the thread is feeding properly.
Are the seams extra thick? Is this new occurrence or steadily getting worse? On my old Singer the feed dogs were not feeding as well as they could and I was able to make a cardboard gasket to go under them to raise them up a bit. This only works on older machines that have visible screws for removing the feed dogs. You may need to bring your machine in for a tune-up and new feed dogs?