There doesn't appear to be any administrator settings for this. (issue raised)
An imperfect solution is to create a wrapper for the cfdump tag by renaming {cfusion}/wwwroot/WEB-INF/cftags/dump.cfm
to (for example) origdump.cfm
then creating a new dump.cfm
file containing:
<cfif isObject(attributes.var) AND NOT StructKeyExists(attributes,'top')>
<cfset attributes.top = 3 />
</cfif>
<cforigdump attributecollection=#attributes# />
<cfexit method="exitTag" />
Fortunately, the writeDump function will call this wrapper (so it works for both tag and function).
Unfortunately the wrapper is not called recursively - if the ORM object is in a struct or array then the original problem still manifests itself - it may be possible to pre-scan complex variables to determine if there are relationships inside and set an appropriate top value, but only a limited solution could be achieved with this (i.e. it would affect neighbouring structs/arrays).