Animals in Morrow project 4th Edition

[WaltShumate]

Could anyone explain this entry on pg 249 of the 4th edtion:-

When resolving damage, death check thresholds should be multiplied by (MASS/10), to a minimum of 0 points for
small creatures. In other words, any damage can force a death check if you are small enough.

I cannot find any other reference to death checks.