Hey good morning @Hunter !
The size of autoflowers can be influenced by several factors, genetics being a primary. When we think of autoflowering cannabis on a consumer level we typically are talking about hybridized cannabis sativa/indica crossed with cannabis ruderalis. Ruderalis itself is typically defined as a short, scraggly plant (low THC, high CBD,) and the telltale visual sign of a large bud on a single stalk. “Ruderal” by definition means “to grow in a waste environment” to give you an idea of why it’s named as such and typically grows in harsher climate/environments (origins believed to be in the Central Asia area and surrounding boundaries where it has adapted to longer daylight hours.)
Some of the better or more progressive autoflower breeders will actually work to remove as much of the ruderalis cross (once autoflowering has been stabilized) and focus on selectively breeding more consumer-appealing photoperiod characteristics, such as larger size, better growth structure, higher potency, etc. The autoflowers we see today are typically much larger in size, more consistent growth patterns, and have potency that scales and compares to many photoperiod strains (and more lab results to start backing up some of those claims even!)
There are some factors that can be limiting for size when comparing autoflowers to photoperiod strains that are important to point out. Autoflowers have a limited life span with a definite vegetative period. They sexually mature and flower out based on their genetics, not light cycle. I strongly emphasize to growers how important each stage of growth is for autoflowers, as it is the optimization of each stage that leads to maximization. Any amount of damage, stress, or repair time induced to the plant is time taken away from optimal growth. Environment and experience of the grower of course also strongly plays into the outcome and result of the grow. The size of the container they grow in will factor in, typically 3-5 gallons minimum is recommended for most autoflowers grown indoors and people are noting filling up roots in containers even much larger (mileage varies.)
There is no comparison in size to a photoperiod plant vegged indoors all winter and thrown outside to finish, but considering the growth, size, and yield we’re starting to see from seed to harvest in 70-90 days average, I do think the work is being put in to give autoflowers a viable place in most growers garden and even gaining some very much potential commercial viability.
I hope that helps Hunter!