Based on what I've read, there
aren't a whole lot of other TXT records at the same nodes where MARID
records need to live.
The bloat problem occurs when you use
wildcards. Since no DNS server I know supports wildcards of the form
_marid.*.phoo.com, you have to put all of your wildcards at *.phoo.com
and that's where you get overlap and bloat.
If specific nodes do get overloaded, SPF and Sender-ID both let you use a short record pointing to a longer record somewhere else as a band-aid.