I play on Gigantic maps as well, I still believe that it would be better for game balance if diminishing returns came into play. I don't think sensor ranges need to be scaled based on map sizes either, because ship speeds are also not scaled based on map sizes. Those two features (what can I see and how fast can I get there) go hand-in-hand.
Example of diminishing returns assuming a 1/4 (or 25%) penalty multiplier for additional modules.
1st sensor gives +10.0 range
2nd sensor gives +7.5 range (17.5 range)
3rd sensor gives +5.625 (23.125 range)
4th sensor gives +4.21875 (27.34375 range)
5th sensor gives +3.1640625 (30.5078125 range)
6th sensor gives +2.373046875 (32.880859375 range)
7th sensor gives +1.77978515625
8th sensor gives +1.3348388671875
So stacking 4 sensors is about the useful limit with a 25% diminishing returns factor. The absolute upper limit would be 4x the base sensor range.
EVE Online ship balancing relies heavily on diminishing returns. It's a useful tool.
As long as you put the diminishing returns value into the XML files, modders could change it to be anywhere from 0% to 100%. A D/R value of 0% means linear improvements as you stack modules. A D/R value of 100 means that a second module is completely useless.
You can retcon it as "too many sensor modules on the ship will interfere with each other".
Note: To avoid exploits where players stack different types of sensor modules, the game should sort all sensor modules by strength before calculating the diminishing returns for each individual module.