650b can be larger or about the same as 700c, but only when comparing apples (700c road tires) to oranges (650b road tires). The concept of road plus kind of renewed interest in 650b by noting that 650x47mm was the same diameter as 700x28mm, so it wouldn’t affect the geometry of the bike, if you have the tire clearance.
However, if you have gravel tires on both wheels, the 700c will be bigger for sure. The difference in radius will be 19 mm at the same tire width, which is 5.6%. In my opinion and experience, that doesn’t matter on gravel. The bigger difference will be in inertia (higher for 700c), assuming you compare similar spec wheels.
Here’s a comparison of tire radii and volume. Volume is proportional to comfort
Tire Size, Radius, Volume
Radius is what affects bb height, handling
700x25 is 340 mm radius, 1.0 L
700x28 is 343 mm radius, 1.25 L, same radius as 650x47 but only 37% of the volume
700x32 is 347 mm, 1.65 L
700x38 is 353 mm, 2.35 L
700x42 is 357 mm, 2.9 L, this is 11 mm higher BB than 650bx50 and 74% of volume
700x44 is 359 mm, 3.2 L
650x42 is 338 mm, 2.7 L
650x47 is 343 mm, 3.4 L, same radius as 700x28 but 272% volume
650x50 is 346 mm, 3.9 L
650x54 is 350 mm, 4.6 L