NOLAfishwater, while line levels in U.S. professional audio are generally +4 dBu (= ca. 1.2 Vrms), consumer line (or "aux") levels are typically referenced to -10 dBV (= ca. 0.3 Vrms).
Professional condenser microphones generally have lower output levels than that. Take the original Neumann U 87--the prototypical studio condenser microphone; its maximum output voltage is only about 200 mV. Driving it to a -10 dBV output level would require about 126 dB sound pressure, which is more than the microphone can handle. Its more recent counterpart, the U 87 Ai, has an even lower maximum SPL capability. 126 dB SPL just isn't a typical studio recording level, though some microphones can certainly handle it; 126 dB is scary loud.
So while it's not an enormous difference and there are definite exceptions (e.g. Neumann's "M 149 Tube" microphone can apparently put out 6-Volt signals, with a nominal sensitivity of 62 mV/Pa in its figure-8 setting), microphone levels in general are lower than line levels in general, to the extent where professional condenser microphone levels are higher than consumer microphone levels, but are still generally lower than consumer line levels.
--best regards
P.S.: If you connect the balanced output of a professional microphone to an unbalanced input by leaving one of the modulation leads unconnected, then depending on the output circuit of the microphone you may well be throwing away 6 dB of signal level, which makes the difference even greater.