To fully appreciate the dynamics of a nuclear explosion, one must understand the scale of the events involved, both incredibly large and infinitesimally small. These can be summed up in four key concepts: time, distance, temperature, and pressure.

TIME
      A microsecond is one-millionth of one second
      A nanosecond is one-billionth of one second
Both of these increments are exceedingly small, yet the difference between the two is amazingly huge. To illustrate, let us consider instead the difference between one million and one billion:
      one million seconds is about eleven and a half days
      one billion seconds is over thirty one and a half years
Many complex nuclear reactions involving astronomical numbers of atomic nuclei occur within an exploding nuclear weapon in a few hundred microseconds.

DISTANCE
Although most fission and fusion stages in a nuclear warhead are located within inches of each other, this distance is enormous when compared to the spaces between particles in an atom:
If a hydrogen atom (the simplest atom, with 1 proton in the nucleus orbited by 1 electron) were enlarged so that its nucleus were 1 inch in diameter, its electron would orbit at a distance of 1400 feet.
Thus, atoms are mostly empty space. The apparent solidity of matter is an illusion, due in part to the enormous number in a small space.
Atoms are so small, that if a drop of water were enlarged to the size of the earth, the individual atoms would be smaller than oranges.

TEMPERATURE
The temperatures inside a nuclear explosion are well over 100 million degrees. The energy at these temperatures is very powerful: it is sufficient to strip a hydrogen atom of its electron, give the nucleus enough velocity to overcome the repulsive force of another hydrogen nucleus, smash them together, forming a new (helium) nucleus and release energy. This type of collision can only occur at extremely high temperatures, and is called a thermonuclear reaction.

PRESSURE
Along with high temperatures, a thermonuclear reaction requires extremely high pressures.
      1 bar equals 98.69% of 1 atmosphere, or 14.5 lbs. per square inch (psi) at sea level
Typical daily fluctuations in air pressure are measured in millibars (1/1000 bar), as seen on weather maps. In an exploding thermonuclear weapon, pressures are as high as a trillion atmospheres--about 8,000,000,000 tons psi.
The result of such enormous pressures on a substance is an increase in its density. The more closely compacted the atoms of a fissile or fusionable material, the more likely they will undergo a collision (reaction), and also a higher frequency of such collisions.
Also, the denser the material, the better (and longer) it can withstand outwardly directed forces which seek to dissemble it, and end the nuclear reactions occuring within it.