Bigger coils and depth readings

I use the Big Foot for large areas of dry sand with my XLT. No way you ever get a foot in depth on a coin size object but even if you could the coil is effectively only a couple of inches wide so the depth reading is a mile out.
None stock coils with any hobby detector don't communicate back to the detector their size so the target depth can't be corrected.
A larger coil (If, IF its able to detect deeper) would guess that the target is at a more shallow depth than it really is. A smaller coil ie Big Foot (because though its 18" in length, its only the width that counts) would indicate a deeper depth than the correct one.