People often think that the biggest difference between the two formats is that medium format uses films that are sharper and better quality. This is untrue. The film/emulsion is exactly the same but it is the larger size of medium format that makes them sharper.

35mm generally has a 1"x1.5" film frame size. Medium format generally has a 2-1/4"x2-1/4" film frame size. Medium format is, depending on the specific cameras compared, three times more surface area than 35mm.

Small enlargements, 4x6 or 5x7, look sharp because the magnification used to enlarge them is small. But when you enlarge to 11x14, for example, you are magnifying that 35mm negative by 10 or 11 times and this is without cropping, if desired. You only need a magnification of 5 or 6 when using the 2-1/4" medium format negative. It is like looking at sandpaper. The farther away you are from the sheet of sandpaper, the smoother it looks. The closer you come to it, the harsher and more "grain" you see. As you magnify a negative to higher and higher magnification ratios, it increases the grain size and thus appears to reduce the apparent sharpness.

So why don't all photographers use medium format or even larger? Size, weight and ease of use. Medium format cameras have traditionally been very big and very heavy. A tripod was your best friend. Not very conducive to documenting fleeting, candid moments. Only recently have camera manufacturers started to produce lighter and more agile medium format cameras.

I use both 35mm and medium format. Medium format is excellent for formal portraits and wider angle views because these types of photographs have so much small detail within them.