Bigger Is Better
The trend in cameras for many years has been towards smaller sizes. Small is good when you want a camera you can carry at all times in a pocket or purse. But, when it comes to image quality, in almost every case bigger is better.
Consider detail resolution. It’s often expressed in megapixels or numbers like 1920 x 1080. There doesn’t seem to be any particular size associated with either; they’re just numbers. A camera with a 4/3-inch-format sensor and one with a 1/3-inch-format sensor seem to have the same resolution if they have the same numbers.
[By the way, there is nothing 4/3-inch in a 4/3-inch-format image sensor and nothing 1/3-inch in a 1/3-inch format. In the old days of video cameras with vacuum-tube-based image sensors, the tubes were measured by the outside diameters of their glass cylinders. Their images sizes were considerably smaller, but, when chips replaced tubes, the tube-size designations stuck).
There are ways of expressing resolution, however, besides raw numbers. One is linear resolution, the number of pixels across a particular distance, usually expressed in terms of line pairs per millimeter (lp/mm).
Two cameras with 1920 x 1080 resolutions have identical numbers of line pairs (960 horizontally). But if a 4/3-inch image sensor has an image width of 18.9 mm, that’s about 51 lp/mm. If a 1/3-inch image sensor has an image width of about 4.9 mm, that’s 197 lp/mm, almost four times as high. And that means the optical system (lens and other components) of the smaller-format camera needs to be considerably better to deliver the same picture quality.
Then there’s sensitivity. Although it’s not really accurate, you can think of each pixel on an image sensor as a tiny solar cell. Now imagine a bigger solar cell. Placed in the sun, it will generate a certain amount of electricity. If you cover half of it, it generates half as much electricity. If you cover 15/16 of it, it generates a lot less electricity. The difference between the pixel area on a 4/3-inch imager and a 1/3-inch imager, all else being equal (which it rarely is) is about 16:1.
Depth of field is a complex issue based on many factors, but, in general, with all else being equal (which, again, it never is), it’s roughly inversely proportional to imager size. That might seem to be an advantage for smaller imagers, and, indeed, if you’re looking for a greater depth of field, a smaller imager will accommodate your purpose better than a larger one. But, if you want to separate an object of interest from the rest of the image by means of sharp focus, you’re probably better off with a larger image format.
There are other advantages to larger imagers, including easier-to-achieve wide-angle lenses, more dynamic range (from noise to white saturation), less contrast loss due to diffraction. And, given the fact that even a full-frame 35-mm image sensor is less than an inch high, bigger doesn’t necessarily mean, well, bigger.