Inline saws, such as wormdrives, are long, narrow, and relatively heavy, while sidewinders are short, wide, and relatively light. Sidewinders predominate on construction sites in the East and Midwest; inline models are about the only saws you'll see in the West. The best explanation I have heard for this phenomenon is from a story in JLC that suggests it's an artifact of distribution patterns from the early days of power tools:

During the depression, construction crawled, but the Army Corps of Engineers adopted the [wormdrive], taking it to Work Project Administration (WPA) sites throughout the western states, and later to hydroelectric and irrigation projects, from which the Skilsaw eventually spilled over onto construction sites. But perhaps the biggest reason the Skilsaw didn't take off in the East was the competition from Porter-Cable [which was well-established in the East and happened to make sidewinders].

These regional preferences remain because the ergonomics of the saws led to disparate methods of use that became the norm in the areas where they're practiced. West Coast framers tend to cut towards the ground, allowing weight of the inline saw to carry it down through the lumber—which they hold off the ground with a foot. East Coast framers tend to work off sawhorses and cut by pushing forward on the sidewinder. Having learned to do it one way, tradesmen have a hard time switching to the other.

From the front it looks like a wormdrive or inline model—minus the cord.

Until the recent announcement of the Makita Rear-Handle Circular Saw (XSR01) no one made a cordless model with the inline configuration preferred by carpenters in the West. There are many reasons why this is so—chief among them the technical challenge of developing an inline saw with sufficient power and runtime to be of use for framing.

On corded inline saws the motor is oriented front to back, with worm or hypoid gears used to turn the axis of rotation 90 degrees to power the arbor that spins the blade. These gears are less efficient than those in a sidewinder—where the axis of the motor and arbor are parallel and it's simply a matter of gearing down to the desired blade speed. Such inefficiency hardly matters in a corded tool but is the kiss of death for one that relies on the finite power stored in a battery.

From the back you can see the motor, which is oriented like that of a sidewinder. 

Makita addressed this problem by equipping the saw with two 18-volt batteries (making it a 36-volt machine) and a brushless motor. Brushless motors are smaller and more durable than their brushed counterparts. And they're more efficient, so they can do more work per charge. The designers did away with the inefficient gears that flip the axis of rotation, by orienting the motor with its shaft parallel to the arbor—which means it's actually a sidewinder. But the form factor of the  XSR01 is that of an inline saw and the manufacturer hopes it will be accepted by carpenters in the west, who until now have had to change the way they cut if they wanted to go cordless.

The manufacturer's claims...

To fit everything in while maintaining the desired form factor Makita put the batteries where the motor of an inline model would normally be and the motor where the gear housing would be. Except for the batteries and lack of a cord it looks every bit the wormdrive or hypoid saw. It's a blade-left machine, bevels 0-53 degrees, and weighs 12.4 pounds with batteries—which for an inline model is relatively light. The saw has an electric brake and an oversize rafter hook, plus something I missed when I saw the tool at The World of Concrete, an opening below the grip that allows the saw to be tethered to keep it from falling.