No. The water flow through the heater core is determined by the total resistance of the system, including the resistance of the valve.
Hydraulically it makes little sense to have two diffeent sizes on the intake line, the very slightly smaller flow resistance of the larger hose d/s of the valve makes miniscule difference.

The reason could be way beyond your imagination, such as:
1. Pre-existing heater core with two 5/8" nipples, change is expensive and not worth it, or:
2. With two nipples the same size either hose could hook to either nipple for easier assembly, or:
3. Running change to the nipple size on the water pump, or:
4. Smaller line adds enough restriction to prevent the water pump from going into cavitation.

See what I mean?

If I had long enough time I could come up with a half dozen more reasons that would be perfectly legitimate to a designer.

R.