My 1st floor toilet hardly flushes and the water comes up in the basement toilet, why?

How we supposed to know? Too big of poops???

Sounds like you may have a clogged vent stack. I had a similar situation. I was able to cut into the vent stack which runs to the roof, in the basement... older house so it was cast metal. Looking up I could not see daylight. I knew there was a clog as I had gone up to the roof and tried to flush out the system with a hose. In my case there had been pine trees around the house at one time. Those needles had entered the stack and compacted to the point that it was like trying to break up concrete. Using a hammer drill and fashioning a 6 ft piece of copper pipe into a spear I was eventually able to break it apart and restore the air flow needed for correct air exchange.

Try using it overnight. You might be surprised. Pour into bow & flush before you go to sleep. Try flushing in the morning.

Sounds like you need to have your septic tank pumped. Generally speaking your septic tank needs a good pumping every now and again.