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The function of a concrete pier is to get a footing down below the influence of weather on the ground . So deeper, within reason is always better.
Cantilevering out of the ground to resist lateral live loads like a telescope mount , again , deeper offers a larger resisting arm. Deeper is still better given the same weight on flat ground.
Creating a bell on the bottom would only be useful in soft ground to spread large vertical loads as a little area makes a big difference.
Having said that the cantilever arm can also be horizontal , so widening the top of the pier with a small slab level with the ground is an easy way of adding more overturning resistance and weight in solid ground with less digging. It would also make use of your reinforcing which other wise is simply stopping the pier cracking around your bolt plates.
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