Both are single aisle, 3 by 3 seating planes. Both are vaguely similar in length (with appropriate models). But they have fairly different ranges.
For similar sized airplanes, why the big difference?
Although the 737 has grown over the years, the 737 and 757 remain in different size classes. Both models can be ETOPS certified, although the 757 is more often used in long flights.
At the time the 757 was introduced (in the 80's), the 737 classic series was current. For the 737-400:
The 757 was significantly larger. For the 757-200:
The 757 is about 35 feet longer, seats about 50 more passengers, and flies about 2,000 nm further (roughly twice as far). The extra range is due to the larger wingspan and longer fuselage, being able to carry more fuel. For this reason, the 757 is used on longer and busier routes, while the 737 is used on shorter, less busy routes.
In the 90's, both models were updated. The 737 was significantly upgraded, including new wings and a new engine, which increased its efficiency. The 737 NG series eventually included the 737-900ER model, currently the largest 737 model.
The 757-300 was also introduced. This was a stretched version of the 757-200.
Although the more efficiency gains of the new 737 bring the range up to nearly that of the 757, the 757 remains significantly larger.
The 757 was designed after the oil shock of 1973 and represented the best which was possible in fuel efficiency. Therefore, it's range is much greater, and with ETOPS certification it can be used on longer overwater trips. Besides, it shares the cockpit layout of it's twin aisle sibling 767, so flight crews could be certified on both with a single training. It replaced the Boeing 727, which is a bigger airplane than the 737.
The 737 is a much older design, but has since then been redesigned twice. The latest models (737-600 to 737-900) were redesigned in the 1990ies and have a new wing and new engines, so their fuel efficiency is again state of the art. Both the 737 and the 757 inherited their fuselage cross section from the 707. With the stretched fuselage of the 737-900, the latest 737 it is almost as big as a 757-200.