Seems that kernel size is not an issue, another thing I've 
noticed is that good and bad kernels have different number of
elf program headers.
Don't know if it matters or not.

Here is good (any kernel that boot successfully has 2 program headers)

netbsd:     file format elf64-sparc
netbsd
architecture: sparc:v9, flags 0x00000012:
EXEC_P, HAS_SYMS
start address 0x0000000001000000

Program Header:
    LOAD off    0x0000000000000100 vaddr 0x0000000001000000 paddr 0x0000000001000000 align 2**7
         filesz 0x00000000001c662f memsz 0x00000000001c662f flags r-x
    LOAD off    0x00000000001c6740 vaddr 0x0000000001400000 paddr 0x0000000001400000 align 2**6
         filesz 0x000000000001c480 memsz 0x0000000000070d10 flags rw-


One which fails to start:

netbsd:     file format elf64-sparc
netbsd
architecture: sparc:v9, flags 0x00000012:
EXEC_P, HAS_SYMS
start address 0x0000000001000000

Program Header:
    LOAD off    0x0000000000000080 vaddr 0x0000000001000000 paddr 0x0000000001000000 align 2**7
         filesz 0x000000000041e7d8 memsz 0x0000000000499768 flags rwx

There is only one program header in this image.

	Andrey