In my mid level practice and budget conscious clients, one of the early discussions is always maintenance costs, and specifically bunkers. As I noted above, bunkers designed for machine maintenance and lowest cost tend to be bigger. Bunkers designed with softer base slopes to minimize washouts tend to get bigger (because a 20% slope takes less room to rise than a 30% slope). That, in addition to matching both sand and grass tongues to machinery. If cost is not an object, those can be free formed for arts sake.
And, let's face it. When bunker liners became almost standard, a lot of bunker design theory changed. At $6-10 per SF (shaping, compacting, drainage, liner, white sand, sodding the banks back) many of us had the same epiphany.....>100,000 SF of sand bunker cost a lot of money, and 50K SF started looking pretty good to owners and supers as well. But, even more than that, with early liners, which required hand raking, instead of 20 foot wide sand lobes for machines, the basic desire was to make those smaller to require less hand raking. If grass noses were going to be hand mown, or left somewhat wild, as is more fashionable now, they can be any size, but also trend to smaller. Thus, smaller bunkers started to make loads and loads of sense.
And, now many gca's really did the "sea of sand" or cluster of bunkers on a regular basis? Steve Smyers comes to mind, although that is as much quantity of bunkers over size of each. I think Tom is right to a degree. Somewhere along the way, just to stand out, someone did larger or more sand bunkers. Eventually, nearly everyone did it, so a lot of large dramatic bunkers no longer started to stand out, so a few different architects started looking for something else to do.