While reviews of Apple Inc.’s (NASDAQ:AAPL) upcoming iPhone 6 are so far largely positive, those of its bigger sibling are decidedly less so. Many reviewers found it "unwieldy" and "uncomfortable," but most had universal praise for Apple's new operating system, iOS 8.

So why didn't most reviewers like the bigger, 5.5-inch iPhone 6? The "extra screen space... makes it unwieldy, and could increase the risk of dropping it," said Charles Arthur from The Guardian, who praised the iPhone 6 Plus' larger battery. Engadget's Brad Molen said the "iPhone 6 Plus is difficult to use one-handed, not to mention more uncomfortable than similar-sized phones."

The Cupertino, California-based manufacturer does not allow users to use multiple apps at the same time, a sticking point for The New York Times' Molly Wood. She wrote that "Apple could have taken a cue from other makers of so-called phablets... and come up with powerful ways to take advantage of those bigger screens."

apple pay Reviews of the iPhone 6 (4.7-inch) were largely positive, but those of the larger iPhone 6 Plus (5.5-inch) were more mixed. Apple Pay, however, is a sure-fire way to extinguish any "Android envy" for iPhone users, according to the Wall Street Journal. Photo: Apple Inc.

The bottom line with most iPhone 6 Plus reviews appears to be that while a double-click feature that pulls the screen down is convenient, the more expensive device is too big for small or average hands.

In regards to the smaller, 4.7-inch iPhone 6, many reviewers claim that once again Apple has developed the "best smartphone" that money can buy. TechCrunch's Darrell Etherington called it the "best smartphone currently available," adding that the iPhone 6 "offers improvements in almost every way that matters, and it delivers those in a striking new design that balances consumer demand for larger screens with a thin, light and durable case."

Lance Ulanoff from Mashable said the camera on the iPhone 6, which lacks the optical image stabilization and a few other features found in the iPhone 6 Plus, was still a marked improvement over the one seen on the iPhone 5S. "I never found the iPhone 5S' autofocus lacking, but when I ran a simple test — covering both smartphone lenses with my fingers and then seeing which could focus more quickly on the scene when I removed them — the iPhone 6 won every time."

The Wall Street Journal's Geoffrey Fowler called the iPhone 6 a "big leap forward in what a smartphone can be in our lives: a real digital wallet" that could eventually become the only "thing you grab when you leave" home. In his review, Fowler called Apple Pay the "ultimate answer to Android envy."

There were a few criticisms of the iPhone 6 as well, notably from David Pierce in The Verge, who wrote that "Apple opted to essentially outline the top and bottom of the phone’s back with small plastic stripes where wireless radios can transmit signal. It just looks bad, like someone drew on my phone with a marker." CNET's Scott Stein added that early battery tests showed that the iPhone 6 "doesn't fare any better than last year's model."

In addition to Apple Pay, the greatest thing that Cupertino is releasing this fall may be iOS 8. Molly Wood called it the "best part of the new phones," as it "combines some of the advanced features of Android with Apple’s ease of use and reliability.”

Additional reporting by Dylan Love