Bigger motorcycles generally tend to be heavier, less maneuverable, and require more skill to manage making them harder to ride overall.
In the same way you probably wouldn’t want your teenager learning to drive via a lumbering three-row SUV, it’s unwise to stick a beginner on the back of a big bike and expect them to have a great time. Larger motorcycles are typically heavier, are harder to maneuver in tight spaces, and usually have taller seat heights, which can make them tough to manage, especially for shorter riders. And, if you accidentally tip one over, getting it upright again can be much more difficult. Big, heavy motorcycles have some benefits, sure, but they require more skill to handle properly.
Let’s compare two bikes from the same lineup to get a sense of their sizes: the Yamaha R3 and the Yamaha R1. The smaller R3 is among several small options from Yamaha that are great for beginners. It weighs just 373 pounds, which is light enough to make it manageable when maneuvering in tight spaces, or picking it back up if you’ve had a slow-speed put down.