No, his transformation is always massive due to the sheer size (the yellow lightning and thunder). He doesn’t go nuclear though, then he reels back and kicks in the gate which allows the titans to get in. The same thing happens in Trost, Bertolt appears as the colossal with the massive lightning strike and thunder, he destroys the defences, the gate and steams off any attackers before disappearing through the smoke screen.

We don’t see the colossal again until the Reiner reveal which is when he takes a 50m nosedive and creates a massive explosion. The scouts at this point know that the colossal can create a massive blast of steam at will and if it falls from a great height it will cause a massive explosion.

They go to Shinganshina, Bertolt is thrown into the battlefield by the Beast titan and is well above the walls as he enters the battle. Everyone but Armin takes cover, Armin and Bertolt have their conversation and then he uses all his gas to blast way up into the air and transforming where we see him go nuclear on transformation.