That is a huge quilt to be running through a domestic sewing machine. It can be done, but I am wondering what batting you used and what model your sewing machine is. Some battings are thinner and lighter, which make pushing the quilt through the arm of the machine easier. Larger arms also make this process easier.
I assume you have installed the walking foot correctly. You can check this by making a little "test" strip of your quilt. Cut a strip of batting, backing and top fabric, baste it as you did the quilt, and then use the walking foot on it. My bet is that this test strip will feed just fine and you will have normal stitches.
If the walking foot is installed correctly, then I am 99.99% certain that the problem you are having is being caused by drag from the quilt. If the quilt is sitting in your lap, the weight of the quilt is enough to produce those small stitches. This is because the feed dogs and walking foot are not strong enough to feed the quilt through from over the edge of your table. This is why you should be lifting up your quilt in front of the presser foot in order to feed the quilt to your machine from above. With a quilt this size, you may also be encountering drag from the sides. You have to make certain, before you sew, that both sides of the quilt are loose enough to not create drag on the feed dogs.
Loosening up the sides and lifting up the front of the quilt will help. You will likely have to stop and do this after every foot or so of stitching.
Ideally, you want to install a system so you can suspend the bulk of your quilt. There are both commercial and homemade systems for doing this. Here are a few links that show you how these suspension systems look. They are usually used for free-motion quilting, but are just as valuable for walking foot quilting.
https://www.youtube.com/watch?v=gJGph8am6BE
http://freemotionquilting.blogspot.c...ang-it-up.html
PVC Embroidery/Quilt support frame
https://www.patsythompsondesigns.com.../archives/5548
http://bryerpatch.com/faq/machinequi...structions.pdf
suspension system for use with sit-down quilting machine