Why is increasing block size in the Bitcoin network considered to decrease security?
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.