Sorry for no replies, I (and I suspect others as well) spent some time pondering this and couldn't come up with much... glad you were able to sort it out.
A bigger battery is always good and quite necessary for starting a diesel engine, but if, with the smaller/weaker battery, you could still start the car by hard-wiring the battery to the solenoid, it sounds like you still have excessive resistance in your starter signal circuit. Ignition switch may have poor contact, or if you converted the car from automatic and the PNP switch jumper connection is not good, you can lose some volts there too. Putting in a stronger, higher-capacity battery will probably be enough to make it work OK, but in a sense it is a patch masking an underlying problem that may cause more problems for you further down the road. If you have excessive voltage drop in that circuit, it won't take much drawing down of the battery before the car becomes hard/impossible to start. May be worth further investigation still if this is a car you plan to depend on for reliable transportation.
Sometimes too, however, the starter solenoids can get weak and won't engage unless they see more volts than should be necessary. Member ngoma and I had a run-around last year with a couple Bosch model SR97 starters that were plagued by this issue. Both tested fine and ran strongly on the bench, but when installed into a car, one of them would not ever engage at all, while the other sometimes worked OK but often would fail to engage or would half-engage and crunch the ring gear teeth. And, both were brand new Bosch OE reman units! After the second bum unit, I got fed up and took it to a good local rebuilder, who rebuilt my shiny new starter with another new solenoid. It has worked fine ever since. The rebuilder said that some crappy or aged solenoids become overly sensitive to the normal small voltage drop that happens in the ignition switch and wiring, so what seems like a wiring issue may just be a tired solenoid. And, hate to say it, but pulling/rebuilding/installing a starter is still easier than replacing an ignition switch! By my reckoning, anyway...
In any case, I doubt if the current is being drawn by the GP relay, since it and/or its little wires would certainly burn up if they were taking enough juice to dim the lights that much. However, even with a discharged battery, you should be able to crank the starter with the key any time that you can crank it with a jumper to the battery. So if this were my car, maybe not immediately but in the long term, I'd probably want to look further whatever issue is causing that difference.
Congrats on getting it running! That is the big step... now the rest is little stuff. Much easier to find the motivation to finish tying up the loose ends and get it on the road after you've heard it light off in its new home.
