The reed side will always be a smaller size because it is thicker. It is thicker for two reasons.
Number one reason is that this part of the coin is thicker. The reed is thicker or raised in order to protect the detail of the coin. As you slide your silver dollar across the bar to pay for that beer, the reed is making contact with the bar and not the “field” of the coin, the detail of the coin. This allows the coin to wear less in order to recognize the coin for a longer period of its life.
The second reason is the forming of the ring. When the hole is punched (in the thinner field of the coin) it is folded and stretched to form the ring. This stretching thins out the metal even more of the cut side. The reed side is compressed and is thickened even more.
This is why is is so important to overstretch your ring past your target size and then reduce the reed side first to hit your target size and then take the overstretched cut side and reduce it to only match the outer look of the reed side making a symmetric ring. If you do not overstretch your ring first, your rings will look conical. The more you overstretch, the more rounded or fat tire look you will get. The less you overstretch, the less rounded appearance you will have. I like 1-2 sizes over for quarters. 2-3 sizes over for half dollars and 3-4 sizes over for dollars. I like a little rounded, not too much.