Any idea of what piece he might be talking about? I seem to remember you were asking about classical sonatas some time back, and that you hadn't done too much for the form. A recording would help...
I have a few thoughts... The different themes might be clunking into each other, not transitioning smoothly, tempo fluctuations. I know I tend to use more tempo fluctuation in shorter pieces, and keep it relatively stable through longer pieces.
How's your theory? in your piece would you be able to point out the different themes, transitions and modulations? What the development is based on... A useful idea I came across from Rosen's book was how tension and release works in the tonal level, with modulations in the sharp direction of the circle of 5ths equating to intensification and modulations in the flat direction equating with a softening. All in relation to the original key. In the large sense of structure, a sonata becomes a rise and climax of keys.
But I think Nicco put it best, as a climb up to the main climax (I'd read some pianists like to call it 'the point' of the piece, heh). There will be all kinds of smaller climaxes in each phrase and groups of phrases, but to try and save the energy for 'the point'. If you have too many high points throughout a piece they all begin to diminish the effect of each other, it loses it's special quality, the point gets lost.
hahah, now that I think of it, that is a bit of a cryptic comment to leave; "CONSIDER THE STRUCTURE..."

. My last examiner left a comment saying I'm good with noodles.