With the geometry of these bikes, the roll-over radius on the front and rear tires need to match each other when the bike is cornered. If the width difference is kept to a minimum between the front and rear, the bike will track on the like you choose, not on its own line.
The front tire is normally slightly smaller width because it carries less weight than the rear. The rear also has (or should have) higher air pressure. When the bike heels over, the tires roll in sync onto their sides: they should deflect the same amount as they do. So, on the 750 as an example, the stock 3.25 x 19 front profile with (old numbers) 28 PSI inside deflects approximately the same as the 4.00x18 rear with 30 PSI inside, and a 150 lb rider aboard. When heeled over, the centerline of the two tire patches then describe the same arc. This is eaily proven out by using a big flat parking lot and a few 5 gallon buckets of water, and by taking a few gentle, steady turns through the water and then going back and examining the resulting wet tire trails on the dry pavement after a pass. If you're dead serious about handling on your particular bike, you will then discover that changing tire pressure 2-4 PSI in either front or rear (usually more PSI) will accurately match up the track, and you will hit a 'sweet spot' that makes the bike feel like it lost 100 lbs somewhere the next time you through it into a freeway on ramp.
All this said, today's tires are largely metric. In the case of the same 750 frame, this translates to a 110-90/19 front with a 110-90/18 rear, with 4 PSI difference (higher in back) with my Avon Road Riders as an example.
The OEM tires were (after 1970) a ribbed front Japanese Dunlop or Bridestone in 3.25x19 with a specially-designed K87 Bridgestone rear, which was designed expressly for the 750. This rear tire had a thicker center tread than on the sides, so it rolled over into a shallower angle to meet the almost-undersize 3.25 front. The handling cure for this situation was to run +3 PSI more in the front than Honda recommended, or else go up to a more expensive 3.50 front tire size. Honda was under the gun too much for costs, so the front stayed at the smaller, less expensive 3.25 tire. We all benefited from switching to 3.50 in the second tire, though.
When the 1976 bikes came out with the 17" rear tire, Honda threw all handling to the wind (it seemed) in favor of trying to boost sales and advertise the extra power of the new engines by showing off the larger rear tire and seriously oversized 630 chain. Both of these features negatively impacted the handling on those later bikes, and many magazines immediately noted it. They can be "fixed up" by going to larger rear rims and more matching tire profiles, though, so not all was lost then, or now.
