I am a bit confused - If it is correct that RL 10 and LR 87-LH2 both were developed in 1950 by Aerojet, and both used LOX and LH2 as the propellants, if so, then why was there a huge difference in their thrust at SL? was there a large difference in their size? or anything else?
There was a large difference in their size, and at the time, they were developed by two entirely different companies.
In general, the propellant combination of a rocket engine does not determine its thrust, just as an internal combustion engine running on diesel fuel may power anything from a lawnmower to a tractor-trailer truck.
The LR87 was developed by Aerojet. It was a large, twin-combustion chamber, turbopump engine originally developed as a kerosene-LOX engine for the first stage of the Titan I ICBM; it was converted to use hypergolic propellants for the Titan II, and then evaluated as a candidate for use with hydrogen as a fuel as an upper stage for the Saturn rocket family. The LR87-LH2 never went into production, as Rocketdyne's J-2 proposal was selected for the Saturn IB and V.
The RL10 was a much smaller expander-cycle engine -- about 135kg in its 1960s incarnation -- designed by Pratt and Whitney for the Centaur upper stage, which was very small by comparison with the Saturn upper stages. The LR87 was over 5 times the mass of the RL10.
Pratt & Whitney merged with Boeing Rocketdyne to form Pratt & Whitney Rocketdyne in 2005; in 2013, P & W Rocketdyne was acquired by GenCorp and merged with Aerojet to form Aerojet Rocketdyne, by which time the LR87 was well out of production.