I know Americans have a reputation for being out of shape, but it's surely not that arduous to carry around an electronic device.
Here are the basic dimensions of the PSP:
17 cm ( 6.69") long.
7.4 cm (2.9133") wide.
2.3 cm ( 0.91") thick.
It's actually only slightly larger than the DS Lite:
13.3 cm ( 5.24") long. (21.6% smaller than the PSP).
7.39 cm ( 2.9094") wide. (.1% smaller than the PSP).
2.15 cm (0.85") thick. (6.5% smaller than the PSP).
In fact, the PSP is smaller than the original DS which was 5.8in x 3.3in x 1.1in.
If you measure volume the original DS was 21.054 cubic inches, the PSP is 17.736 and the DS Lite is 12.958.
Weight? Original DS was 275g (9.7 ounces), the PSP is 260g (9.17 oz.) and the DS Lite is 215g ( 7.58 oz.)
All in all, yes, the DS Lite is smaller. But to say that it's significantly smaller or that the PSP is too large to carry around is simply nonsense.