Why is increasing block size in the Bitcoin network considered to decrease security? I have been doing some research on the Bitcoin Cash hard fork and the main contention of increasing the block size appears to be the possibility of less security on the network. How does an increased block size result in a less secure network?
Generally speaking, a larger block leads to more computational resources (tx validation, bandwidth, storage, memory) required for each person who wishes to validate newly confirmed transactions.


Higher validation cost lead end-users to rely on/trust
centralised services to "validate" their transactions.
Larger blocks require more time to propagate in the network, 
increasing pooling pressures for more centralized mining-pools.


The lower the validation cost, the more we can push validation to the edge (end-user) of the network, the more decentralisation we can achieve. Decentralisation is ultimately the source of security, as it is harder for an external force to attack.