For robustness, growing is helpful but not sufficient. You need to reproduce.
However, reproduction is not merely making copies. That is barely different from growth. Again, redundant structures and information help, but they're not sufficient.
To survive longer periods and greater risks, you need to duplicate and separate.
The bigger you are, the further you have to separate.
Your "size" is not your mass, it is the space you occupy. If you are frequently highly mobile, that is like being large, and means you have to separate further.
There are two ways to separate: either you use a different mechanism of movement for separation than for all other purposes, (like a plant seed blowing on the wind), or you make a sustained determined effort to escape, to run far away from all of your kind, with a high speed and consistency of direction that you do not use for any purpose other than separation.