For at least the last 10-15 years some stores have been running the sneaky “mark up the bigger box” scam since they know most just assume the cost per unit is cheaper and grab it.
Don’t forget... all manner of sins shall be forgiven thee except the unpardonable sin... paying full retail or overpaying. Lol
Here in Indiana, grocery stuff has price per unit on shelf tags, sometimes pretty small but usually readable.
And you’re right, sometimes a large box of cereal is more per ounce than a smaller one.