First: What Win32diskimager does is creating a bit-to-bit copy of the SD card. This means it copies every bit, even if it stores no data.
There are several tutorials how to to that on a Linux machine, like this one https://raspberrypi.stackexchange.com/a/56312/71377
Furthermore there are automatized scripts to to that on Linux, e.g. https://github.com/qrti/shrink.
I don't know a way to handle this problem on Windows (which you seem to be using) properly. I would recommend you using a Linux VM (virtual machine) or Linux live system which can be booted from a USB stick.
To do so, download e.g. a Ubuntu image from here. Copy it to a USB stick the way you copied Raspbian to your SD card using Win32DiskImager. Boot your PC from this USB stick and click on "Try Ubuntu". Now you have a working Linux environment which can be used to shricńk the image following the guides I linked above.