1. Partition the smaller key
2. Format the partitions
3. Install bootloader to the smaller key
4. Copy contents of the bigger key's partition to the smaller key, preferably via tar to keep permissions
For a one-off thing, you could just do it manually instead of scripting.
If you're not familiar with partitioning/bootloaders, just install to the smaller key via tc-install a default system, then replace the partition contents with those of the bigger stick.