Now, comparing Bioplasm to Psychatog is like comparing the best thing on Earth to a piece of garbage you picked up at the dump.
The main difference is, with Psychatog, you feed it and it's guarenteed to get bigger. Your opponent won't know what size Smilie will be before he blocks. Does the player have a Thirst For Knowledge or Fact or Fiction to give it more cards to feed? Or is it simply the number we see now? Or will he just keep it a 1/2 if it's unblocked?
With Bioplasm, you have to have it charge into the enemy and play a game of risk. Will the card you reveal show something large enough for your creature to be bigger/better then anything your opponent has in play? Or will it hit nothing and be a simple 4/4?
Your opponent will see how large your Bioplasm is before he blocks, as removal of top deck is done upon declaration of attack. This means if you reveal a 5/5, your opponent can easily just drop a 1/1 to chump it thanks to the lack of evasion. And if you reveal a instant, sorcery, or land, your opponent can just drop a their 5/5 to take it down. And when that happens, not only did you lose your 5 mana 4/4, but you also lost a potentially playable card that was the unfortunate victim of Bioplasm.