Shares of Potbelly (NASDAQ:PBPB) soared 9% yesterday. Investors were clearly impressed by the sandwich shop's first quarterly report as a public company. 

Adjusted earnings climbed 27% to $0.15 a share, well ahead of market expectations. Revenue climbed nearly 12% to $78 million, narrowly ahead of what the pros were projecting. 

A beat on both ends of the income statement may typically warrant a stock's pop, but this is a stock that has doubled since going public at $14 last month. This is the kind of move that should accompany a blowout report, and this wasn't exactly a perfect quarter.

We saw fellow fast-casual rookie Noodles & Co. (NASDAQ:NDLS) take a hit last week after it reported its first full quarter since going public this summer. It grew even faster than Potbelly, with revenue and adjusted earnings increasing 15% and 45%, respectively. The market wanted more. Why is it settling for less with Potbelly?  

It's important to note that the growth at oven-heating Potbelly and pasta-boiling Noodles & Co. consisted largely of expansion efforts. Same-restaurant sales actually inched just 2.1% higher at Noodles & Co. in last week's report and 2.5% higher at Potbelly in yesterday's announcement.

Neither of these chains is the next Chipotle Mexican Grill (NYSE:CMG). Noodles & Co. and Potbelly roared out of their debutante gates because of Chipotle's success. The market isn't interested in eating up fast-food chains or traditional casual-dining establishments. However, there's something appetizing about the "fast casual" niche that offers the best of both worlds, with table-service-quality eats served at the speed and convenience of fast food.

Chipotle showed us earlier this earnings season why fast casual matters. Despite being much larger -- it has five times as many restaurants as Potbelly -- Chipotle still managed to grow revenue and comps by 18% and 6.2%, respectively, during the same three months.

If it wasn't for Chipotle's success, the market wouldn't be eating up Noodles & Co. and Potbelly now. As refreshing as it was to see margin expansion on an adjusted basis at both chains, retail investors wouldn't have jumped on the IPOs if their same-store sales were barely keeping up with inflation. One can even argue that if larger sandwich shops were public -- and it's odd that Subway, Quiznos, and Jimmy John's are not -- the market wouldn't care about Potbelly unless it started sporting Chipotle-esque growth.

It is encouraging to see Potbelly exceed the low bar that analysts had set for the sandwich maker. You definitely don't want to fall short as a recent IPO, shredding credibility in your first chance to impress. However, when you're trading at 76 times next year's projected profitability, it's natural to hold out for a little more bread.