I agree.  I can see Stacey's point about FOV, but it does nothing to
explain why the Olympus Zuiko Digital 300mm f/2.8 ED is no smaller or
lighter than 300mm f/2.8 lenses for 35mm cameras.

The size issue appears mystifying but there is actually a very good
reason for this.  The reason lies in the basic differences between a
lens designed for film and a Zuiko Digital lens.

Zuiko Digital lenses are designed to be as near telecentric as
practically possible for each focal length/aperture - or range of
focal lengths/apertures in the case of the zooms.  A lens that is
purely telecentric means that all the light rays exiting the rear
element arrive perpendicular to the plane of the sensor.  Lenses that
are designed to be telecentric will almost always be much more bulky
and weigh far more than lenses that are not.

This also explains why the lens throat in the ZD mount is so large.
It is much bigger than the Nikon F lens throat and almost as large as
the Canon EF throat, despite the fact that the Four Thirds sensor is
only about a quarter of the area of the 24x36mm frame of 35mm film.

The small Four Thirds sensor would appear to indicate that lenses for
that system could be physically significantly smaller and therefore
much lighter than lenses for 35mm film - but they are only smaller and
lighter than telecentric lenses for film would have been, if they
existed.

To put it another way (one that hopefully makes more sense) Olympus
chose from the outset to offer lenses for the Four Thirds system that
are as near telecentric as possible.  This means that part of the
potential size and weight advantage of using a smaller sensor has had
to be given up in order to obtain the optical characteristics that the
designers strove to achieve.

The near-telecentric design of the Zuiko Digital lenses helps to
explain their extremely good optical performance.  The pro grade ZD
lenses are extremely sharp even into the corners at wide apertures,
inviting much favourable comment from testers including many
comparisons with Leica glass - high praise indeed.  

This characteristic sets them apart from most other lens ranges for
DSLRs.  That is because no other manufacturer has yet optimised their
lens designs for use with digital sensors (rather than film) to the
extent that Olympus has.  Indeed, other brand lenses "optimised" for
smaller-than-24x36mm sensors have mostly been optimised to reduce
bulk, weight and cost with little or no attention paid to making them
telecentric.

Of course the fact that Olympus started with a blank sheet of paper
has helped here.  With no legacy 35mm range to accommodate, Olympus
has been able to set priorities for optical design that are far better
suited to digital capture.

This goes a long way to explaining why Zuiko lenses for the Olympus OM
System perform so badly on Four Thirds.  The results are pretty dire,
except for a small number of OM lenses used at a specific, usually
small range of apertures (and focal lengths, if zooms).  That is
because most Zuikos for OM are far from being telecentric designs.

Exactly the same applies with other brands of lens when they are used
on Four Thirds bodies using third party adapters.

I should point out that the inexpensive Zuiko Digital kit lenses for
the E-300 are not nearly as telecentric as the pro range, which
explains their smaller size, lower weight and low cost.  They also
perform less well, but because they are still designed to be as near
telecentric as practicable at the price point, they are still very
good performers.

I hope this all goes some way towards shedding a little light on a
discussion that seems to be taking place in near-total darkness.

Because no one expects the same image quality. I