Think out of the box.
First of all, I use the standard size of Esbit tablets and I have never seen the small size in any store. Sometimes I am cutting them in half to boil a single cup of water. I stand the half-cube on its end, not flat. That slightly changes the distance from flame to pot.
Now, I suspect that leads to a faster boil time, but slightly less fuel efficiency. However, when I am in a situation where I am using a bottle cap, I am generally in a hurry.
Normally I use my Esbit fuel in an Esbit 3-wing burner. Again, I can burn it flat, or on its side, or on its end.