Observations: The crystals grown in the fridge are needle-like and the bottom of the jar is filled with them. The crystals grown on the counter are bigger and smoother and squarish and there are few of them in the jar.
Conclusion: My hypothesis was correct, temperature does affect the growth of epsom salt crystals. But the crystals are smaller.
Why?: The heated water molecules move farther apart, making room for more of the epsom salts to dissolve. When no more of the salts can be dissolved, you have a saturated solution. As this saturated solution cools, the water molecules move closer together, slowly pushing the salt out as a solid, creating crystals.