I had not much of a clue about the answer to your question, until I realized the significance of mentioning Planck time.
Here’s my guess: Presuming that everything about Planck units is correct, then nothing can be physically smaller than one Planck length, and duration can be quicker than a Planck time. If so, then at the Big Bang, we first had nothing, and then, one Planck time later, we had Something. But that Something’s size was only 1 Planck length in any direction. Not enough time had passed for it to grow bigger in any direction.
Now, take my last sentence, and instead of the word “direction”, substitute “dimension”. At 1 Planck time, the Big Bang was not too big yet – only one Planck length in each dimension. Remember that time is a dimension too. And now that the universe is capable of length and width and height, it’s capable of duration too.