Think if it in car terms. Which is the better car, one with a speedo that goes to 160MPH or one that goes to 200MPH? Does any of that really matter if they are both limited to 155MPH? (elect or mech, doesn't matter.)
A larger bios simply allows more features to be added, but you still need the company to make the features. If the bios comp A makes is only 2.23MBs large, it doesn't matter if its in a 4MB or 8MB bios. I've seen some companies use the extra bios space for things like including Memtest as a bios option. Again, it only matters if whoever made the motherboard bothered to put anything extra in the bios/chip.