Final answer:
A functional mRNA in the cytoplasm of a eukaryotic cell is smaller than the primary transcript in the nucleus due to processing steps like splicing which removes introns.
Step-by-step explanation:
A functional mRNA in the cytoplasm of a eukaryotic cell is smaller than the primary transcript found in the nucleus. This is because eukaryotic primary transcripts undergo several processing steps before becoming mature mRNA. These steps include the addition of a 5' cap and a 3' poly-A tail, as well as the removal of non-coding sequences known as introns through a process called splicing. After splicing, only the coding sequences called exons remain, which results in a functional mRNA that is shorter than the original primary transcript.