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A GL1500 is considerably larger than a GL1100 or GL1200, and there is more room for rider and passenger. The GL1800 not so much, since it’s smaller than a GL1500. I rode GL1200s for many years and now have a GL1800 as well. The wife says that there is really not that much more room on the back of the GL1800 for her as there is on our ’87 GL1200. (She’s small, however, so that may have something to do with it.)
Honda did take 2 inches of fore-and-aft room from the rider portion of the GL1800 seat and give it to the passenger when they designed the GL1800 seat. They did that at the request of their passenger demographic, which are typically a good sized people. Large riders have a problem on GL1800s since this puts their knees too close to the fairing. Many buy aftermarket seats to fix that issue.
If you go with a GL1200 I would strongly recommend getting a 1987 model. That year had a great seat redesign! It was a one year only seat for the GL1200s. The seat was made flatter, with no crown in the middle (from side-to-side), and the passenger seat had a slight upward angle toward the front to keep the passenger from sliding forward. These changes made the seat WAY more comfortable than any previous Gold Wing seat. I had a new ’84 years ago, and as soon as the ’87 came out, I bought an ’87 seat and put it on my ’84. Big difference. Wife said it made a world of difference for her as well. My current ’87 has a stock ’87 seat as well, and it’s a great seat. Also have a trunk relocation kit on the ’87 to move the passenger backrest back a couple inches. This gives everybody more room.
To answer your original question "How much more roomy is a GL1200 compared to a GL1100?" Not that much really! But a seat changecan make a big difference. Maybe all you need is a new aftermarket seat for your GL1100 and the trunk relocation you are planning.