Why are mRNA sequence so much smaller then a Gene sequence

i understand that a gene is double stranded, and mrna is transcribed from a single strand and also that introns are removed. But are these the only reason why the sequence length is so different in size.

Example a gene is 20142 and the mrna is 1021.

Most of that size difference is due to introns, as you correctly mention. When you mention the "gene sequence", do you also include regulatory elements that are not transcribed, like the TATA box, Shine Dalgarno sequence, enhancers, etc., or are you referring only to those parts of the gene that are transcribed? These non-transcribed regulatory elements add many nucleotides to the DNA sequence that would not be in the pre-RNA or mature mRNA.

After the primary RNA transcript is made from the template strand of the DNA, it also contains the non-coding sequences called the introns. The introns are later removed in a process called RNA splicing and exons are joined together. This produces the mature mRNA that only contains the exons. Take note that the total length of introns on a eukaryotic gene is much greater than the total length of the exons. Hence, when introns are removed from the mRNA, it becomes significantly shorter than the original length of gene.
I hope this helps.

mRNA is a Messenger that has to be able to be transported through Membranes, so it has to be small in order to fulfill its purpose. To achieve this it only consists of the Information necessary for its particular task. Introns are not necessary for mRNA since it needs no buffer material of any kind as it is produced for a particular purpose.