We present ～0.6″ resolution IRAM PdBI interferometry of eight submillimeter galaxies at z ～ 2-3.4, where we detect continuum at 1 mm and/or CO lines at 3 and 1 mm. The CO 3-2/4-3 line profiles in five of the sources are double-peaked, indicative of orbital motion either in a single rotating disk or of a merger of two galaxies. The millimeter line and continuum emission is compact; we marginally resolve the sources or obtain tight upper limits to their intrinsic sizes in all cases. The median FWHM diameter for these sources and the previously resolved sources, SMM J023952-0136 and SMM J140104+0252, is ≤ 0.5″ (4 kpc). The compactness of the sources does not support a scenario in which the far-IR/submillimeter emission comes from a cold (T < 30 K), very extended dust distribution. These measurements clearly show that the submillimeter galaxies (SMGs) we have observed resemble scaled-up and more gas-rich versions of the local universe, ultraluminous galaxy population. Their central densities and potential well depths are much greater than those in other z ～ 2-3 galaxy samples studied so far. They are comparable to those of elliptical galaxies or massive bulges. The SMG properties fulfill the criteria of "maximal" starbursts, in which most of the available initial gas reservoir of 10~(10)-10~(11) solar mass is converted to stars on a timescale ～3-10t_(dyn) ～ a few times 10~8 yr.
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