Why splitted text files is bigger than a large one with the same content?
Filesystem overhead.

Every time you create a file, the filesystem reserves space on disk to store the file name, and other metadata like its permissions, creation date, etc. It also has to maintain a sort of index of all the (possibly sparse) blocks occupied by each file. The index might be a simple list or a tree, depending on the filesystem.

Finally, note that most filesystems can not allocate only a portion of a block for a file, so the allocated space needs to be rounded up to the block size. This rounding affects both the file blocks, and the index blocks.