I'll take a stab at answering your question.
Notice that Y axis motion is limited by the column of the mill. You might be able to get more Y travel by cantilevering the head/spindle outward a longer distance, but that would reduce stiffness. Starting out with the center of the table under the spindle, the "forward" width of the table is usually somewhere near where the spindle will be if the table is cranked all the way back to the machine's column. As for forward table motion, there's no real need to move the table beyond the point where the spindle aligns with the rear table edge.
Now consider the X axis. You could theoretically make the table as long in X as you wanted to, with accordingly long travel. The practical limiters here are (1) the strength of the table and its supporting structure ... don't want the table to sag from a heavy load on one end (or cause the mill to tip over sideways) when it's cranked all the way out. (2) the practicality/cost/precision of extremely long lead screws. (3) Shop space ... The clear horizontal area needed by a mill is roughly twice the X length of the table.
For machines like 3D printers, where the X and Y mechanisms are overhead, supported by posts at the corners of the working envelope, you can make the X and Y travel distances as long as you want to (given whatever limits there are to accuracy, stiffness, etc.)
Hope this is useful to you.