The difference in size of the mintmark (if any really exists) would have NO bearing on whether you would have a large or small date, if those exist. Mintmarks and the hubbed design were completely separate and independent processes.
As to whether you have discovered something new, I do not yet know, but am willing to investigate. It's "probably" die wear, but every lead is worth looking into. I'll let you know what I find after having a chance to look through the 1977 cents I have here.
BTW, if you did discover something new, there will be other changes - not just the date. There will be differences in relief, differences in the bust, and differences in LIBERTY and the motto too. History dictates that when they change the date size, they change the entire design - not just the date.
Also, you need to have at least undamaged AU coins to compare. Comparing 30 year-old coins you found in circulation isn't a good measure because of wear and other things that would be hidden on the coins. Find some 1977 coins at a dealer or a show that have your different sizes and use those for reporting your find. I'm probably the ONLY one who would bother looking further into it from a report using damaged circulated coins.
As to whether you have discovered something new, I do not yet know, but am willing to investigate. It's "probably" die wear, but every lead is worth looking into. I'll let you know what I find after having a chance to look through the 1977 cents I have here.
BTW, if you did discover something new, there will be other changes - not just the date. There will be differences in relief, differences in the bust, and differences in LIBERTY and the motto too. History dictates that when they change the date size, they change the entire design - not just the date.
Also, you need to have at least undamaged AU coins to compare. Comparing 30 year-old coins you found in circulation isn't a good measure because of wear and other things that would be hidden on the coins. Find some 1977 coins at a dealer or a show that have your different sizes and use those for reporting your find. I'm probably the ONLY one who would bother looking further into it from a report using damaged circulated coins.

















