Sizing air conditioning by square footage of conditioned space is barely better than a WAG. That ASM chart and their cheesy square feet x (pick your constant) divided by 12,000 is complete & utter crap! Cooling loads are a function of exterior surface area and solar gain, not the square footage of the interior space. That's why a Manual-J is called for.
^^pure BS^^
Do the Manual-J! According to that silly chart my house should have a load of 4 tons, when in fact it's under 1 ton, and is not well served by the totally insane 5 ton central air conditioning previously installed. It's oversized to the point of being nearly useless for the latent loads, so I use a half-ton window-shaker in an upstairs window to cool the whole house, except at temperatures well above the 1% outside design temps, where I might have to run the central air for exactly ONE cycle per day during a heat wave to keep the place cool.)
The fact that an oversized AC system can keep the house cool should come as no surprise- it has the capacity, but it it's not particularly optimal, using more power than needed, making more noise than needed, and costing too much up front. It's pretty typical to see oversizing factors of 2-3x, (rather than the 5x at my house) but you'll be more comfortable if it's no more than 1.5x oversized for the 1% design load.
The very fact that your biggest cooling bills are $150/month is evidence that it actually
IS oversized, possibly by quite a bit! If it was appropriately size it would be running a much higher duty cycle, and using much more power than that.
You can prove it by actually
measuring the duty-cycle on the compressor with a data-logger the next time the outdoor temps are in the range of your
1% outside design temperature (about 90-93F for most of FL). I suspect you'll be pretty surprised at your oversizing factor. But if the thing is only 4 years old, you'll probably be using it for quite awhile longer, and there's no "payback" in swapping it out for a more appropriately sized system as long as it's keeping you reasonably comfortable.
Without knowing the time of day that picture was snapped it's hard to say for sure, but it looks like you have pretty good mid-day to afternoon shading factors and very little west facing window area, which would lower your peak cooling load compared to a typical house that size with less-favorable shading factors & window orientation.