I had big babies too (both 20lbs by 4mos old!) and I never used an infant tub. I had one for our oldest and used it one time and decided it was a huge pain in the butt! I bought a big sponge at Wal Mart for $3, and used it for laying down baby, sitting up baby, and moving baby. It was nice to have somewhere to plop his butt when they started sitting up and were still slippery.
Its yellow, shaped like a bear, and in the baby isle at Target or Wal Mart for $3-5. I had one for each boy, and I always buy the mfor friends now. Way cheaper, easier, and longer lasting than a baby tub!