With MyISAM is there any index size savings when using INT vs BIGINT?
Decided this was easy enough to investigate myself, even though I still do not understand the underlying specifics of why (feel free to elaborate?)

The answer is:  YES  int most definitely creates smaller indexes than BIGINT

I made two tables, first with four unsigned INT columns, second with four unsigned BIGINT

I made a compound index across all four columns for each table.

Then I added a million rows of random unsigned smallints 0-65535  to each table.

(each table has identical data, both numbers and row order)

Then I optimized and flushed both tables just to be certain.

INT

Data    17,000  KiB
Index   31,610  KiB
Total   48,610  KiB


BIGINT

Data    33,000  KiB
Index   56,921  KiB
Total   89,921  KiB




17,408,000 int.MYD
32,368,640 int.MYI

33,792,000 bigint.MYD
58,287,104 bigint.MYI




added:

I was concerned the random data repeated (I found some cases).

So I added a primary column with auto-increment to each table and emptied them. Then I filled each with the numbers from 1 to 1,000,000 in each column, incrementing for each row sequentially. 

INT

Data    20,508  KiB
Index   32,301  KiB
Total   52,809  KiB


BIGINT

Data    40,039  KiB
Index   54,178  KiB
Total   94,217  KiB


So not quite 50% savings but definitely adds up, even for index storage.