The 737 icon is significantly bigger than the Airbus icon. It should be the other way around, shouldn’t it?
You gave no context so I assume you are talking about the dump1090-fa icons.
There is no specific icon for the A321; there is an A320 icon that is used for the A319/320/321 family.
There are many 737 variants; the icon is based on a 737-800.
A320 is 35.8m by 37.6m
737-800 is 35.8m by 39.5m.
And the icons are not necessarily to scale; but if they were to scale, the 737-800 icon should be larger.
Much of this is documented in markers.js.