Yes its the screw size,you can get 100 watt bulbs and 100 watt equivalents in both E14 and E27 sizes. go and have a look on amazon, you don't have to buy there but it will give you an idea of what is available. Put E14 bulb in the search engine.
"E" stands for Edison fitting, IE Screw in type.
14 stands for the diameter of the fitting 14mm
27 stands for the diameter of the fitting 27mm
Bulbs of any wattage are available in either size.
Take care fitting a higher wattage bulb than what the lamp is designed for.
An E14 screw is about the thickness of your little finger. An E27 screw is about the size of a "normal" bayonet light buld. You cannot screw an E14 bulb into an E27 socker or vice-versa.
The wattage is nothing to do with the thread size, except that the E14's will only come as low-wattage and the E27 may be higher.
You need to look at the size of the socket in the lamp and buy the correct screw bulb.
Bear in mind the EU regulations are now (or will soon) banning the supply of high-wattage bulbs - you will have to go for energy-savers if you want very bright light.
Can I suggest LED (bulbs....although they're not really bulbs) because they use very little power, stay cool and you can get very bright ones. They cost more but last donkeys' years and pay for themselves in vastly reduced power consumption. Look here to start with...
gingebee's suggestion to switch to LEDs is a good idea.
I've been slowly switching to LEDs myself. I agree they are more expensive and you have to match the output in Lumens to what you had in the oldstyle bulbs. I'd suggest you use 'warm white' for a more natural look.
LED bulbs are great - We've got a few in our house. The problem is that you don't seem to be able to get 100 watt equivalent and the OP seems to be looking for a very bright light.