Depths in different sized molds

I am pouring liquid into molds.

My original molds were 1.5"x1.5" and had a depth of .75"

I poured 3mL into each mold, and the liquid did not come near the .75" depth--I don't know how deep the liquid was once it was poured in, maybe like 1/2 or 1/3 or something.

I now have different sized molds, but I want to keep the depth of the liquid I pour the same as when I poured it into the smaller molds.

How do I figure out the volume of liquid to add to the larger molds so that the depth of the liquid remains the same?

Thanks!