More likely is that the prospect of hurting Russia too bad and it falling apart is one of the key drivers of the hesitation for more support.

This was the line of reasoning:

• Too little, Ukraine loses and Russia becomes enboldened and might try again.
• Too much, option 1: Russia escalates and starts lobbying nukes.
• Too much, option 2: Russia fractures from and internal powerstruggles and splits into several chaotic warring nuclear powered states.

None of these options look appetizing.