Q: “I’ve heard that light arrangements often transfer better than thick arrangements for radio play. Is this true, and why?”
A: I’m sure that this is a subject that could be argued both directions, but it is true that dense, complex arrangements are more likely to be turned into “mush” once they’ve been put through the dynamics processing on the radio. Having said that, there are a number of thick arrangements that survive the radio just fine (Queen comes to mind, as well as Phil Spector productions and others), but in many of these cases the vocals are mixed and mastered well above the arrangement, and the arrangement themselves are voiced well.
The key to constructive arranging, such that all voices are intelligible, is to be aware of the various frequencies (and timbre, etc.) of the voices so that there is enough “space” for everything. While the song itself is arguably the most important component of any hit, and you always want to shoot for a compelling (even inspiring) arrangement of instruments, becoming familiar with the differences between the frequencies, timbre and tone of say a violin versus a flute will mean the difference between writing arrangements where they’ll both get heard and not “walked on” by each other – which happens often with these two instruments in particular.
So, while a light arrangement might transfer better, a good arrangement where all voices have their own space should fair just fine. Of course, this assumes a great mix and master.