You should always try and see if the man page (
man mke2fs) is more informative than the help:
Specify the bytes/inode ratio. mke2fs creates an inode for
every bytes-per-inode bytes of space on the disk. The larger
the bytes-per-inode ratio, the fewer inodes will be created.
This value generally shouldn't be smaller than the blocksize of
the filesystem, since in that case more inodes would be made
than can ever be used. Be warned that it is not possible to
expand the number of inodes on a filesystem after it is cre‐
ated, so be careful deciding the correct value for this parame‐
Specify the size of each inode in bytes. mke2fs creates
256-byte inodes by default. In kernels after 2.6.10 and some
earlier vendor kernels it is possible to utilize inodes larger
than 128 bytes to store extended attributes for improved per‐
formance. The inode-size value must be a power of 2 larger or
equal to 128. The larger the inode-size the more space the
inode table will consume, and this reduces the usable space in
the filesystem and can also negatively impact performance.
Extended attributes stored in large inodes are not visible with
older kernels, and such filesystems will not be mountable with
2.4 kernels at all. It is not possible to change this value
after the filesystem is created.
To me this reads as that with
-I you specify a fixed size which is independent of the disk size (which might be inappropriate for your disk's size). And with
-i you set the number of inodes, which takes into account the disk's size.