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No, you aren't crazy. As others have stated mileage often drops a lot with new tires. Not only is the first 1/32 of a tire generally a gripper compound but the more tread, the more tread squirm, the more heat generated which equals lower fuel economy.
It also seems you when oversized - which as redpoint said will increase speedometer error and understate both speed and miles driven. This will be even greater if your last set of tires were smaller than stock.
Then there is the change in gearing. Larger tires = lower RPM which could put the engine out of the efficient powerband and make it lug more at lower highway speeds.
Then there is the fact that MPG is a poor unit to measure fuel economy because it is not linear. The higher the base MPG the less fuel saved by each additional mpg.
Driving 15,000 miles a year:
30 mpg = 500 gallons per year
60 mpg = 250 gallons per year (Saving 250 gallons)
90 mpg = 167 gallons per year (Saving 83 gallons)
120 mpg = 125 gallons per year (Saving 42 gallons)
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