It's not just about the size of the map but the length of common angles. Both classes of sniper should try to use their maximum range as much as possible. If a map doesn't have a lot of places where a Splatterscope can't reach, then go ahead and use Splatterscope. If there are several dominant angles that only an E-Liter can achieve then you'd be giving up a lot of potential if you pick Splatterscope.
For example, Kelp Dome seems like a fairly small map, but the length of the open area from the center to the edge of the map can only be abused by E-Liter. And that route sees very high traffic so you do want to abuse it. E-Liter can also reach between top mid and either spawn, where a Splatterscope would have to climb a ramp or block where they can easily get locked out. However, kit is also a consideration, and splat bomb rush, killer wail, and splash walls are all very strong on kelp.
On the other hand, Blackbelly doesn't have any common angles that can only be achieved by E-Liter. That map favors mobility, switching between very situational angles following the movements of your enemies. I'd pick Splatterscope there. Even if you're camping top mid, you may not be able to reach as far into their base, but you can still cover all the choke points out of it.