Saying that the large numbers you can handle with 64 bits allow for better encryption is misleading. 64 bits is too short for modern cryptographic algorithms.
Those algorithms which do rely on large numbers, need numbers much larger than 64 bits. For those, the computation have to be split up and processed in smaller parts. But being able to process 64 bits at a time rather than 32 means you can reduce the number of instructions needed by a factor of 2-4.
Symmetric algorithms, which don't work with large numbers but rather with multiple variables each sized conveniently for machine words, things may be different. For example the SHA2 algorithm can work with either 32 bit words (SHA-224 and SHA-256) or 64 bit words (all the other SHA2 variants). All the variants can be computed on both 32 and 64 bit CPUS. On a 32 bit CPU, the 32 bit variants are faster than the 64 bit variants. On a 64 bit CPU, the 64 bit variants are faster than the 32 bit variants.
In the end the most significant challenge to cryptography on a smartphone, is how the user can enter a password or a crypto key with sufficient entropy through the limited user interface. The type of CPU doesn't change that challenge at all.