Seven segment display becomes dimmer as more segments are lit
My first assumption from the symptom is that you have a single cathode resistor, which would explain everything.  However, you say this is not the case and that you have individual 100 &Omega; resistors on each anode line.

The answer then must be that whatever is driving the cathode can't handle the combined current.  Apparently you are driving the cathode with just a microcontroller output, which has insufficient current sink capability.  Do the math.  You want 10 mA thru each of 7 segments.  That means whatever is holding the cathodes low has to be able to handle 70 mA.  A microcontroller pin is very very unlikely to do that.  See the datasheet.

The solution is to add a transistor that can sink the current from each of the cathode connections:



70 mA is easy for any discrete transistor you will be able to get.  Let's say the transistor has a minimum guaranteed gain of 50.  That means the base current needs to be 70mA / 50 = 1.4 mA.  Figure the B-E junction drops 700 mV.  With 3.3 V drive from the micro, that leaves 3.3 V - 700 mV = 2.6 V accross the base resistor.  2.6 V / 1.4 mA = 1.9 k&Omega;, which is the absolute maximum allowed base resistor.  Given that, I'd use 1 k&Omega; between the digital output and the base.

Note that this will invert the common cathode logic from what you had.  The microcontroller output now has to go high to enable a digit.