There are two kinds of transits: squadron jump and standard transit.
In a standard transit, one jump drive can handle an arbitrary number of ships as long as they are all smaller than its max size (i.e. 18kt in the current example).
In a squadron transit, one jump drive can handle a fixed number of ships (3 is the starting tech, can be increased at a slight increase in size of the drive). All ships ALSO need to be <= max size (again, 18kt).
The proposal is to modify squadron transits to work on a total tonnage capacity, rather than a number of vessels. So a 3x jump drive at 18kt could jump 3x18kt ships, or 6x9kt ships. But not 1x36kt ship + 1x18kt ship.