i think you'll get a similar response from most people, but
calves are an easy exercise to butcher, as far as form.
definitely re-check your form, in fact, go on youtube and watch calf raising training videos...
you admit yourself you have skinny calves.. even guys with great calves don't use that much weight. doesn't that make you skeptical?
the achiles tendon is a very strong tendon with great elasticity. that's commonly why calf exercises can be cheated so easily as you can just bounce on that tendon, and 300lbs it may be, it'll still bounce.