I have a watch-sized computer. It works for me.
I will observe that if you can't see a watch-sized computer you probably can't see your SPG.
Never having used a watch-style computer, I can't help but wonder how important that lesser bulk under water will be for you. What are the concerns about a larger computer? Do you expect your arm to become sore during a dive because you are carrying so much more weight? Will it be too hard for you to squeeze through tight passages? Will it get in the way of taking your gear on and off under water? Will you become too fatigued from lugging it around to get back on the boat after the dive?
I would agree that the smaller size of a watch-sized computer offers few if any useful benefits, for scuba diving, while in the water.
I find that the main advantage of a small computer is that it can be worn during other activities. Since my computer is the single most costly piece of dive gear I own, I find it useful to keep it on my wrist before and after dives. My diving has evolved to become one of many water sports I participate in, often on the same day. I like to leave the computer on my wrist while freediving, canoeing, swimming, or boating or on the beach before, after, or in between scuba dives. I usually do not have a cell phone immediately available in these situations and sometimes need to know the time, and for freediving, my depth.
I not only carry a relatively large computer, I carry two of them. I don't notice any problems related to bulk. I like the information I can see at a glance from those large screens. I like the ease of operation from buttons I can manipulate even with thick gloves.
I find that I can operate the buttons on my Oceanic Geo 2.0 with 5mm gloves, without difficulty.
I do not anticipate that deep dives will ever be a major part of my diving. My computer is mainly there to tell time, temperature, and depth, and to log these data. While diving, I only have to push a button for temperature.