'Could' suggests that there is an ambiguity in the amount of food the person ate. In other words, it may or may not have been a large amount (after all, this varies from person to person).
'Can' suggests that the person definitely ate a lot of food.
I think it's difficult to say this without context (as usual).
Here is a scenario:
"I ate three hamburgers, four large fries, three desserts and a few milkshakes. You could say I ate a lot at lunch." Here, there is no ambiguity at all. But it would be wrong, in most cases, to say, "You can say I ate a lot at lunch."
Here's another scenario:
A: "Your mother is on the phone and wants to know if you're eating well."
B: "You can say I ate a lot at lunch." (Tell her I ate a lot at lunch.)
Can and
could are used in their normal way in these sentences.
"You could say ..." is an idiom, meaning roughly: "Without describing everything in detail, it would not be wrong to say ..."
