More fluid volume in the heart makes for more pressure, but that doesn't make the heart pump faster. The (normal) myocardium responds to increased stretch (like from increased filling volume) by generating an increased force of contraction. That's where the higher cardiac output comes from. The blood pushed out with each contraction is called stroke volume.
An athlete's heart is big, because it's routinely asked to do big work, like any other exercised muscle. Increased venous return from exercise fills it up, and it generates a huge force of contraction and a big stroke volume. But when there's no particular need for a big cardiac output (of, say 12 liters per minute to run a marathon) when sitting or resting, the heart rate goes way down, because a big stroke volume can generate a normal cardiac output (4-5 liters per minute) with fewer strokes. This is why athletes have resting heart rates in the 30s and sometimes even lower. When their heart rates are high, they generate a terrific cardiac output to support the athletic endeavor of their choice. Then at rest, the high rate isn't needed to support ordinary activity. (Stroke volume can also increase with adrenalin, because it makes the ventricle contract even harder. Separate issue.) (Heart rate, volume, and cardiac output in heart failure is a whole 'nother chapter, but suffice it to say that too MUCH venous return for a sick heart to handle causes force of contraction to go DOWN rather than up, thus cardiac output drops, heart rate may increase to try to compensate for falling BP, and bad things happen. Ask the Magic 8 Ball about this later.)
What does this have to do with cardiac output in pregnancy? Well, if you have a bigger stroke volume because you have a bigger blood volume, you don't need a higher heart rate to supply an adequate BP. I'm not sure that pregnancy automatically confers a lower heart rate (I'll defer this to the OB/GYN experts here), but your initial assumption that more volume requires a higher rate is misconceived (as it were). Does that help?