I would not be surprised or consider it problematic if the size of the output of show run varied between firewalls, especially when they are running different versions of code. With different versions of code there will be differences in what features are supported which might impact the amount of output. And it is likely that the ASAs are configured differently. For example one ASA might have multiple site to site VPNs configured each of which needs crypto ACL, crypto map, tunnel group, NAT/NAT exemption which would contribute to differences in amount of config output.
HTH
Rick
HTH
Rick