When one marks Split Lap time, the split time is shown on the Forerunner screen in a small rectangular box. In the background one can see that the original screen is running but the rectangular box and split time hides it all so there is no need to show this.
I think it would be much more beneficial to use the maximum possible font size to show the split time when the split button is pressed (this split time only is shown for about 5 seconds anyway) as this is information that is observed during the run and the bigger the numbers are the faster this information can be assimilated.