Question: In general I have found that as the focal length of 35mm (or APS-C) lenses increases, the physical length of the lens increases as well. However, my large format lens is much thinner than other 35mm lenses of equivalent focal length. Here is a picture:

[![Two lenses](https://i.stack.imgur.com/oNMHx.jpg)](https://i.stack.imgur.com/oNMHx.jpg)

The large format lens is a Wollensak Raptar 135mm f4.7, and it not more than 5 cm long. The lens next to it (albeit slightly unfair as it is a zoom) is a Sun telephoto zoom, 85mm-210mm f3.8. In general all large format lenses seem to be rather diminutive in length. Why is this?

Answer: 
The focal length is the distance from the (theoretical) center of the lens to the image plane. On the large format camera, there's a lot more *camera* between the lens and the film.

The lenses are also often relatively simple — there's no need for a focusing mechanism in the lens itself, for example. 

@osullic gives the example of the [Schneider PC TS Makro-Symmar 90mm f/4.5](http://www.bhphotovideo.com/c/product/818360-REG/Schneider_06_1064382_PC_TS_Super_Angulon.html) for 35mm and [Schneider 90mm f/4.5 Apo Digitar N](http://www.bhphotovideo.com/c/product/400328-REG/Schneider_03_017780_90mm_f_4_5_Apo_Digitar.html) for large format. They appear to have similar optical design (six elements in four groups), and this is particularly interesting, because the former is a *tilt/shift* lens. Tilt lets you change the angle of the focal plane for depth-of-field effects; shift changes the appearance of parallel lines. As with focus, tilt and shift are both adjustable in large format by adjustments *to the camera*.

So, basically, with 35mm, much of what would be in the camera in large format is *moved to the lens*, which of course makes the lens bigger.
A few points to consider (mostly adding to mattdm's answer):

A manually focusing 135mm Nikkor 2.8 lens in F mount is about 91.5mm long, and looking at a drawing of the lens most of the optical elements are in the front. So a comparison with a zoom lens isn't really fair -- it is a much more complicated lens.

Strictly speaking, a telephoto lens is one designed to be shorter than its focal length, otherwise a 500mm lens on an SLR would be mostly a big empty tube with optical elements at the end. The telephoto design allows the lens to be shorter but requires more optical elements. In a view camera this type of construction isn't normally needed as the camera bellows can expand or contract to suit the needs of a wide range of focal lengths (and focusing distances).

See also:

<https://en.wikipedia.org/wiki/Telephoto_lens>

<http://www.mir.com.my/rb/photography/companies/nikon/nikkoresources/135mmnikkor/135mm28.htm>