NFPA 211 is the standard for solid fuel burning appliances and venting. That is then adopted into codes. The standard is where you'll find the decreasing up to one inch allowance. I don't believe anything is mentioned about increasing. It is just detrimental to the efficiency and will cause rapid creosote build up by decreasing flue gas temperatures as the gasses expand. When temps goes below 250*f. the water vapor condenses on the flue walls allowing smoke particles to stick creating creosote. So increasing certainly becomes a safety factor if the operator isn't aware of it. You also need to leave much more heat up to create the same draft you would have with the smaller diameter. A waste of fuel. One line in the entire book of the safety standard states all appliances must also be installed as per manufacturers installation instructions. That way if the board that writes and approves the standard isn't aware of something a manufacture proves safe through testing, they are covered. That is where the wording in the correct manual comes into play since the manual is part of the UL listing. For instance when a manufacturer changes something in a manual there is paperwork involved to approve the change with U.L. Since yours isn't a listed appliance without a tag, this doesn't pertain to yours, but if you had a listed stove, to maintain the listing, it would technically be required to have "ONLY 7 inch C.S.A. or U.L.C. Listed factory built, residential type and building heating appliance chimney." that is the words from page 6 of the Canadian manual and notice the listed parts are tested to Canadian specs. This is why I'd like to find a manual for the U.S. market. They are then uploaded to the manual section for others free of charge. Installing as per instructions guarantees the appliance is installed "as tested". Sometimes specifications given in manuals are for different reasons. Such as being tested with a 15 foot chimney. The manual then must state to use "15 foot minimum" which many fear the stove will not work correctly if they have a shorter chimney. It has nothing to do with the operation or safety being shorter, it was just tested with that standard height and needs to be that minimum to be installed "as tested". Once you understand where codes come from and read them yourself it will come together. I always stated "no reduction in flue diameter was allowed" until bholler corrected me that the standard states reduction of up to one inch is allowed. I have the paperwork from Fisher and most of the stoves, others here have details about codes that affect their business and experience from what they specialize in working in the heating industry. I'm retired from the heating industry so my information is outdated compared to the newer appliances, but it puts me in a good position for collector and antique stoves before the information is lost forever. The reason for safe clearances is due to drying of building materials over time at elevated temperatures. The lower the moisture content, the lower the ignition temperature. When someone says "but it has been fine for xx years that way", that is time to be concerned.