VCO is for "voltage controlled oscillator"
Charge is transferred from charge-pump into loop filter after integration, to form voltage to VCO. If your VCO input is not having infinite input impedance, charge will definitely leak and VCO input range will be degraded. Or put in another word, when the charge pump delivers current to loop filter (charge pump has finite output impedance Ro), if it sees another impedance from VCO, they will form a voltage divider which limits the VCO input voltage range.
Output cap is always need to small enough for having wider and higher operating frequency range as more cap, lower frequency can oscillate