Every day your beans are a day older. The temperature has changed, the humidity has changed, the barometric pressure has changed, your dose has changed, your extraction temperature has changed, etc.... You will need to make small adjustments to the grind to compensate for the aged of the beans and changes in atmosphere. Your dose is also varying shot to shot unless you are using a scale to weigh every shot. In short, a lot of things change. A very large cone could also be a dead spot in the middle of the puck. No espresso flowing from the center so the 'cone' does not extend to the center of the basket which will also change the size, and could explain the bubbles since you have a dime sized air pocket under that cone. Make sure your flush is correct, verify your dose shot to shot and you probably need to tighten your grind just a bit between day 1 and day 4.