Apologies for brevity, noob here and just throwing out an idea in case it's
useful (probably already covered somewhere, but I haven't got time to do
all the necessary background research).

From https://github.com/bitcoin/bitcoin/issues/13342

Suggestion:  To make it more difficult for a malicious attacker to reap
quick rewards by double-spending large amounts with a relatively brief
majority of the network hashing power, introduce a hash workload that is
proportional to the sum of transactions in a block (probably the sum of the
absolute values, and a "proportionality function" could be linear or
exponential).  The motivation is to make it more difficult for malicious
attacks to hash-power their way through a few large transactions.
Obviously, there are costs in greater transaction delays (and fees?) for
larger amounts (absolute value).

If there is original value in the idea, I can try to make time to follow-up
with a better BIP proposal.

-- 
Darren