Besides there being the ATtiny45 and 85 chips for the uno etc, are there any bigger chips that work the same with the Mega?
And if so, would it be able to program the chips as the uno does or would it need more to achieve it?
Was just wondering as with the higher capacity of the mega.
No with the appropriate programming adapter an uno can program a bootloader into anything.
Depends on what you mean by Bigger.
There are a whole lot of different Attiny's and Atmega's which can be made to work with Arduino.
ATtiny45 and 85 chips for the uno
I don't understand the question, what do the 45/85s have to do with the Uno?
If you mean can the Uno program chip X then as Docedison said yes, and so can the Mega but there's no relationship between Uno/Mega/bigger/smaller etc, either the Uno or the Mega (or any other Arduino I suppose) can be programmed to program any other AVR chip.
ArduinoISP is just a piece of the puzzle. Other workstation such as AVRdude is also involved. So the scope of comparability is a bit broader than the U I-Mega2560.
A bit dated, but a good Overview: Installing an Arduino Bootloader - learn.sparkfun.com