The HPROF binary format "JAVA PROFILE 1.0.1" limits records to 4GB. The agent should be updated to support the new tags defined for format "JAVA PROFILE 1.0.2". To keep compatability the agent should continue to generate 1.0.1 format for smaller heaps (<2GB for example).
The binary HPROF format (using format=b) option contains a field that is 32bits wide that provides the byte length of the heap dump records. This size does not map simply to the size of the java heap, but observations have it matching the java heap size pretty closely. In any case this format=b format (1.0.1 version) will not accomodate large heaps. The 1.0.2 hprof format=b layout added a segment opcode that allowed for the larger heap dump to be segmented. The textual dump may have limitations with regards to size also, however this has not been investigated fully.