To make an oscillator you need 'gain' to overcome the component losses and 'phase shift' so the feedback is in the right polarity. To achieve this inside the 8051 there is an inverter (providing the gain and some of the phase shift) wired across pins 18 and 19 inside the IC. The crystal and the two capacitors provide the remaining phase shift. The frequency is decided primarily by the crystal which will only allow feedback at one frequency (at least in this simple circuit). The capacitors load the inverter and crystal to complete the resonant circuit. The frequency will vary slightly (a few KHz at most) if the capacitor values are changed but the crystal provides almost all the frequency control.
If the capacitors are made too large the overall gain will be lower and oscillator will take longer to start up. Beyond a certain value the oscillator will not start at all.
If they are made too small, the crystal, the 8051 and the circuit board become the dominant capacitances and all of these are less predictable and more prone to changing with temperature.
The suggested values are a compromise that satisfy the fast start requirement and give good stability.
Brian.