star wars - What objects can & can't penetrate ground based shield generators?

At the battle of Hoth - shields had to be lowered to allow a transport to leave.

However, apparently AT-AT's, which are large, were able to walk through them (unless they got inside the shields when the shields were lowered to let a transport leave - which isn't stated anywhere).

Battle droids walked through ground based shields at the Battle of Naboo.

Just what is the logic - is there a size limit? Can small ships penetrate shields? If so why don't we ever see that and why can't larger ships? If not - why can AT-AT's and droids?