Why is it important for the transcription bubble to remain a constant size?

The process of the formation of DNA into RNA is called transcription. The site of transcription occurs in the cytoplasm of prokaryotes

Initiation, elongation and termination are the three steps of the transcription. The promoter, a gene region, is where initiation occurs when the RNA polymerase enzyme attaches to it. Nucleotides are added to the mRNA strand during elongation. When RNA polymerase passes a stop (termination) sequence in the gene, transcription ends or is terminated.

Initiation of transcription requires the formation of a transcription bubble. The promoter regions in contact with the RNA polymerase holoenzymes are identified by a process named footprinting. In this process, DNA is incubated with a protein to which it is bounded and is then treated with an alkylation agent such as Dimethyl sulphate.Dimethyl sulphate is the residue of $N_7$and A residues at $N_3$in both double and single-strand DNA. DMS also methylates $N_1$of A and $N_3$of C. But involved in base pairing interaction.

Thus, the transcription bubble resembles the region of unwound DNA at the replication origin.