Several readers commented about the small size of the D5100. Attached are few images comparing the size of the D5100 to the D700.
The D5100 (5 x 3.8 x 3.1 in | 127 x 96.5 x 78.7 mm) is slightly smaller than the D5000 (5 x 4.1 x 3.1 in | 127 x 104 x 80 mm). The weight of the D5100 and the D5000 is the same (19.8 oz | 560 gr). The D3100 is still the smallest and lightest DSLR in the current Nikon lineup (16 oz | 455 gr | 4.9 x 3.8 x 2.9 in | 124.46 x 96.52 x 73.66 mm).
The Nikon D5100 uses the EN-EL14 battery that promises 660 shots (the EN-EL3e battery from the D700 is rated at 1,000 shots). The D5000 was rated at 510 shots (CIPA):
The Nikon D5100 doesn't come with a power cord: