There is no set-in-stone standard in the fast food industry for what constitutes a “small,” “medium,” or “large” drink, so sizes will inevitably vary from eatery to eatery. But if a company sells you a “22 ounce” soft drink, it best come in a container that can hold that amount of fluid. However, this doesn’t seem to be the case at Arby’s.

Consumerist reader Michael recently bought a small drink from an Arby’s in Ohio. And printed right on the Arby’s-branded paper cup it clearly states “22 oz.”

But then Michael noticed some text on the underside of that same cup that states “21 oz.”

Since you can’t put 22 ounces of liquid into a 21 oz. cup, Michael busted out the old measuring cup to confirm that the cup does indeed hold the smaller volume of liquid.

Michael says he’s not terribly upset about the shortchanging — after all, 21 oz. is still quite a bit of drink for a small size — but it does bring up the question of how widespread this particular apparent mislabeling might be, and how long Arby’s has been selling drinks in these particular cups.

To see if this issue was relegated to just the franchise visited by Michael in Ohio, we sent a Consumerist reporter to buy a small soda at an Arby’s in Arlington, VA.

Lo and behold, these cups also stated 22 oz. on the outside of the cup and 21 oz. on the underside. And the measuring cup test confirmed that the Arby’s cup could only hold 21 ounces.

We also looked at other sizes of drinks available from Arby’s, but only the “22 oz.” cups were different from the size printed by the manufacturer on the underside.

When reached for comment on this issue, a rep for Arby’s would only tell Consumerist, “Thank you for bringing this to our attention. We are looking into the matter.”

Fast food customers already get less than they pay for at the soda fountain thanks to the huge volume of ice used to water down most soft drinks; no company should be using mislabeled cups — which could be in violation of the law — to give customers even less value.

We have brought this story to the attention of the offices of the Attorneys General for Ohio and Virginia to ask which, if any, state laws might apply to the labeling/size of fountain sodas and where consumers in these states can go if they believe they are being shortchanged. If we hear anything back, we will update.