Can you put smaller jetting in your natural gas furnace?

No... that would be underfiring it, causing the inside of the furnace to become a dangerous, sooty mess. If you are looking to save fuel I would say the proper way to go is to have a Manual J heat load calculation done on your home and then if indicated switch to a smaller or more efficient furnace. Furnace size is indicated on the mfrs unit tag as BTU Input (fuel required to produce stated output). The BTU Output rating is the rated Output after allowing for inefficiency losses (what is lost up the chimney). For example a furnace with an input rating of 100,000 BTU and an output rating of 80,000 BTU is an 80% efficient furnace. Supposing your Heat Load calculation says an 80,000 output is what you need, you can save by switching to a 90% efficient 90,000 Input furnace which will have an Output of 81000 BTU/hour (90,000 x 90%) same Output/less Input (fuel). Input is the fuel usage rating, ie Natural Gas provides 1000 BTU/hour per cubic foot used. Therefore with the 100,000 BTU/hour Input furnace, you will use 100 cubic feet of gas for every hour it is running, You can see this on your gas meter, First note the BTU input of your furnace from the tag, divide that # by 1000 (1000 BTU/hour per cubic ft) then divide the result by 60 (60 minutes in an hour) = X cubic feet. Then with the furnace being the only gas appliance in the house operating watch your meter for 60 seconds and if everything is operating properly the reading you get will equal X.Then do the math in reverse X times 60 times 1000 and you should have the Input # you started with. Now going back to the 100,000 80% furnace vs the 90% efficient furnace, it is easy to see one would save 10 cubic feet of gas per hour of run time by upgrading to the 90% or better. In my neck of the woods that equates to a savings of $1.28 for every 10 hours of run time.