OK.... from what tire to what tire (brand/series)

Different series different brand has different tire characteristic. Some tires have softer sidewall and some harder. Even if you put same air pressure, the one with softer sidewall will bulge more thus the overall diameter of the tire is less than the harder sidewall.

With smaller diameter, the tires need more rotation to go the same distance compare to the other tires leading to more fuel used. The more contact, the harder to roll, adding another factor. Another possibility is that you are comparing your old end of life tire with new one.. The old tires lost probably about 2.5 cm of radius, thus this contributes to at lease 3% of fuel consumption increment.