342M isn't that big. My T-5140 servers, running Solaris 10 with Sun Cluster, have boot archives in the range of 355M to 375M.
boot_archive file is like the linux ramdisk image. It contains a kernel image, all the kernel modules, and several configuration files. It's read during the boot process, prior to / being mounted. Once / is mounted, the ramdisk image is discarded, and the real files are read directly from disk.
If you're curious about the contents, the boot archive is a gzipped ISO 9660 filesystem.
cp /platform/$(uname -m)/boot_archive /tmp/boot_archive.gz
lofiadm -a /tmp/boot_archive
mount -F hsfs /dev/lofi/X /mnt # <- change X to your lofi dev number
and explore. you'll see exactly what's taking up space.