How much your throttle butterfly is open at cruising, what is angle of throttle butterfly?
I would consider it being kind of wall that stops so much of pulses that it has no effect, also often there is 90 degree direction change with intake runners and plenum.
Problem often comes when intake piping is long and still original diameter, which makes it more restrictive, longer you go, larger diameter it must be to not be more restrictive.
Full power is another thing again, it is hardly ever used on street except with young ones.
I have not seen any negative effects of bigger exhaust at dyno (not any gains with otherwise standard vehicle), well except noise. Surely everything is best made so that there are balance, but of course with everything one must seek balance and optimal solution, as with everything going too far is going too far.
With exhaust more effect than size is what kind of silencer boxes there are, often those are such that gas turns 180 turn twice before exiting, especially with standard family cars, making distance gas travel further and also any turn is restriction even it would be made smooth, which sadly often is not the case.
I got 0.5l/100km gain with one of my cars that had silly back box where gas did enter from same edge than it did exist, then pipe did go around the box something like 270 degree turn after it made 180 degree turn at box and it was quite sharp turns straight pieces between them. I put see trough box there, it was bit loud for sure after that, but there was also some gain. Maybe some of it was because it was easier to judge engine load from exhaust note, maybe it was so loud that I did use lower throttle subcutaneously, who knows, I was not much to hypermiling then, I met rev limiter daily
One could of course make exhaust larger, found this silly pic from the interwebs
