Extent is easiest to measure. Weight is the most difficult ~ until we get the ground penetrating radar down tight and then we can figure that out by measuring acceleration at depth ~ dirt and rocks move aside due to the weight of the ice.
I've only seen one good one so far and that was the use of ground penetrating radar combined with other methods to take a look at the tectonic plates sitting on top the plume that provides the heat for the magma that fires up Yellowstone. Geology had that one within the last year I believe ~ kept it near the family throne room to make sure I could read it all.
Let's say Maximum ice is not all that PrecICE.
Which brings up something else ~ the archeology shows two thrusts ~ which makes sense ~ if you have Maximum Ice and giant ice flows are putting out to see you might not be able to cross the Atlantic. Both before that time and after that time you should be able to do that ~ particularly if you are adapted to eat vast amounts of seal meat.
I don't know it you've ever looked closely at a shaded relief map of America del Norte or not, but in particular around the southern edges of the Great Lakes you'll find what appear to be numerous "terminal" moraines. That would indicate to me that most likely the ice sheets' "flows" became smaller and smaller as they gradually retreated in fits and spurts overall. Maybe not but that's the way it looks to me...