It has been well-documented throughout the series that both Dan and Roseanne have gained a lot of weight since high school. At the beginning of this episode, Ziggy even comments on how big they have gotten. During the rebuilt motorcycle reveal, both Dan and Roseanne put on their leather jackets from high school. Dan's fits just right and Roseanne's is maybe a size too big. If they were both much thinner in high school, these jackets would have been unrealistically large on them, which is not how leather motorcycle jackets would have been worn.
When Ziggy and Dan go out to the garage to look at Dan's old 1955 Harley Davidson Panhead. Dan pulls off the cover to reveal a trashed pan head chopper. After they rebuild the motorcycle, it suddenly becomes a blue and white 1990 Harley Davidson Softail Heritage. Later in the series, the motorcycle changes to a 1991 Softail with a different paint scheme (cream and seafoam green).