I'm not sure about that big chunk of extra pad on M1, but if you just mean the other memory chips have 60-70% covered then that should be OK. It's not ideal for a thermal interface to only cover part of the chip being cooled but the heat will move to the area with interface material. Also, memory chips are less thermally sensitive than processor chips which is why you don't see active cooling on desktop memory modules.
One thing to look for is the shape of the actual sink on the other side. Sometimes the actual sink only covers part of the chips for whatever reason, so even if you had pads covering the whole chips it wouldn't make a difference.
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