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The bigger pack is cheaper per unit. Except when it is not, which is more often than you would expect.
Why this is worth checking
Shops know that shoppers assume bigger equals better value, and that assumption is strong enough that the larger size does not always have to earn it. This is not a conspiracy; it is just that nobody checks, so the discipline slips.
The cases where the big pack loses are predictable: promotional pricing on the small size, odd pack sizes that are hard to compare mentally, and anything sold in both weight and count.
Cheaper per unit is not always cheaper
This is the part the calculator cannot do for you.
Will you use it all? A better unit price on something that expires is not a saving. Halve the quantity you will realistically use before comparing.
Storage costs something, even at home. Buying six months of anything bulky is a real trade for space.
Money now versus money later. Spending more today to save later is only sensible if the money is not needed elsewhere first.
The comparison that actually matters
Not "which of these two is better value" but "do I need this at all right now?" A 40% saving on something you did not need is not a 40% saving.
The calculator is useful once you have already decided to buy. It cannot help with the decision before that one, and that is the decision worth more money.