Walking the coil and listening closely once you know the machine will tell you if the object is large or not. ....
Yes. Sure. If the objects are spaced apart. Then yes a guy can deduce large vs small, deep vs. shallow, etc.... Just based on sound. But if you're in a relicky-ruins/ghost-townsy environment (where the surface is a continuous blanket of targets), then you're screwed. Because , for example, a bunch of pulltabs, foil wads, nails, and zincs, all crowded together 1/20" apart, can mimic a big target. Since they are a seamless continuous "target", so-to-speak.
And this is the reason yesteryear machines like the old BFO's, and all-metal TR's (94b, 77b, 66TR, etc...) actually found more caches than are being found today (on a per capita basis of hobbyists out there detecting). Because BY THEIR VERY NATURE they were insensitive to small objects. Lacked ability in minerals, etc... So for example, a BFO might have only attained 3" on a coin-sized object. Yet did PERFECTLY WELL on a soda can or hubcap. Doh!
So you see: "More sensitivity" is actually counter-productive to cache hunting. At least when that added sensitivity starts to meld itself to ability to find little things. If they're all strung together in junk-yard fashion, you will spend all day and night beating your brains, and
"digging a bunch just to be sure". Whereas a 2-box simply doesn't see anything smaller than a soda can.