It helps to lift up the quilt sandwich so you are feeding it to the foot from above. Also make sure there is no drag from the side.

I have to admit that I don't know the answer as all but one of my quilts have hand binding. The one that I tried machine binding on I found it difficult to do the corners. Could it possibly be the speed you are going that makes for the different size stitches?

Sometimes this happens when I get to a thicker part of the binding, like at a seam where the binding strips were joined, or a seam in the piecing. I find that I can just kind of help push the piece through at a steady rate to avoid it slowing down and giving me smaller stitches.

I do all my bindings by machine. this happens if I have drag on the quilt. So I pull up about 6" of the quilt and have it loose in front of me, then sew that 6" until the slack is gone, then pull up another 6" so it's slack and repeat. At no time do I sew so that the quilt is being pulled up from hanging down off the table.

I also have to watch that I'm not accidently pinching the fabric with my body parts. With the quilt partially in my lap, when I lean forward sometimes it gets tucked under by "girls" and then won't feed freely under the machine. As soon as I feel any drag at all, I stop and reposition so there is no drag. I also have a sewing stool next to me to help support the weight.

I do a lot of my sewing very late at night and often on a deadline. Both of these issues can be troublesome. I found that often when I need to do a lot of straight stitching (binding or hemming or attaching bias trim), I use a decorative stitch and often with a variegated thread. It looks like I was adding a 'design element' when I was actually trying to cover any possible wobbles! Makes everyone happy.

To me, it looked like the tiny stitches happened when there was more bulk under the foot. If this was a pieced quilt perhaps they were just bulkier intersections [on the other side of the quilt].

I was surprised to see how much lint packs in the valley of those feed dogs. Check them out.

My machine does this when the fabric hangs up on the edge of the table or something else.