Carom cues have what we call a conical taper or straight taper. It gets big fast. The reason is rigidity. Carom balls are larger & heavier than pool balls, and it's common to send the ball many cushions. Pool cues with long pro-tapers are much less rigid because of flex. Due to human hand size dictating that a comfortable tip size is usually somewhere between 12mm-13mm, that means the only way to strengthen the cue up is to beef up the taper.
Considering that carom existed before pocket, I'd guess that modern taper shapes began as a means of increasing accuracy & finesse as the cue is stroked. The straight taper of a carom cue makes it tough to judge where you contact the cue ball. You think you're hitting bottom for a draw shot, but by the time the tip contacts the ball, the taper has brought the tip up, creating less draw than you thought you were going to get. A pro-taper allows the tip to travel parallel with the table surface through the stroke. A carom taper doesn't.
That's probably more answer than you were looking for, but this subject hits home with me
This exact difference was partially the reason I make cues. I was in Korea for a year & played a lot while there. I'd play carom games at the pool rooms & pool at the bars. Considering that carom was the popular game, bars had carom cues as house cues even though they were being used for pool. At the time I had a Meucci. One day I decided to try my Meucci while gambling on a carom game. It was nearly impossible to control the larger ball as it felt like a bowling ball, and the ferrule soon failed. Being a machinist, I found some white plastic & repaired the cue. It changed everything. It was like a totally different cue. Still no good for carom, it hit & felt totally different when playing pool. The fascination of
"why" is the seed that grew into me being a cue maker. Much of my philosophy in cue making is based on the notion of marrying the no surprise point & shoot accuracy of a carom cue with the fine control of a pool cue. I probably wouldn't be making cues today if not for spending that year in Korea. And if not for learning the difference between a carom cue & pool cue, the cues I make today certainly wouldn't be what what they are.