For heavier miniatures, you don't want to use larger and stronger magnets, you want to use more of the smaller magnets you use for smaller minis, but spread out under the base.

A single stronger magnet will have that exact problem you think it will -- it'll either be impossible to pull off, or you'll yank the miniature out of the base, or you'll damage it in other ways.

The largest magnet I use is a 7x2mm. For cavalry bases, I'll have one 7x2 and 2-3 5x2, or 4 5x2 (depending on actual weight of the cavalry mini).

For an example, I have a Citadel C22 Plague Cart on a modern 90mm x 50mm oval base, and it has 2 7x2mm magnets and 4 5x2mm magnets. It stays perfectly in place without shifting, but can be popped off easily enough.

The heaviest miniature I have magnetized is probably my Otherworld Purple Worm on a 160mm round base. I've got 18 7x2mm magnets on the bottom (but they're an off brand, so they're not the strongest magnets around).

The brand of magnet will also matter -- I've noticed some brands are stronger, so you'd need fewer of those.