Many journals publish correspondence or short reports that are brief research works, usually with a single finding, straightforward methods, and not much more than that. They are intended for quick, "Hey, we've always wanted to know the value of X, and turns out it's 7" studies - they belong in the literature, and may be useful, but are not a full research paper.
They are often also referred to as letters, notes, or by other names. Sometimes they're purely defined by concept, sometimes by word count.
For example, consider Ecology:
"Notes are short papers that present significant new observations and methodological advances. Notes may contain results that are not sufficiently elaborated or developed as to justify an Article, but which are still of considerable potential significance."
"...an Article tells a more complicated story with distinct components. The greater length of Articles relative to Reports must be justified by their greater complexity
Or American Journal of Epidemiology, which uses word counts:
The maximum number of words per article, exclusive of tables, figures, references, and abstract, should be as follows: Original Contribution, 3,500; ...Brief Original Contributions, 2,000 (with no more than 2 half-page tables and 40 references)
or Infection Control and Hospital Epidemiology:
Original Articles should include a title page, a structured abstract of no more than 250 words (see below), a text of no more than 3,000 words, no more than 7 tables and figures, and no more than 40 references.
Concise Communications should include a title page, a narrative abstract of no more than 50 words, a text of no more than 1,200 words, no more than 2 tables or figures, and no more than 10 references.
Research Briefs should include a title page, a text of no more than 900 words, no more than 1 table or figure, and no more than 10 references. This category of article is intended for the presentation of short, focused, and evidence-based experimental observations: substantial preliminary and novel results of importance to the journal readership but not substantial enough in content to warrant a longer presentation. Research Briefs undergo the same peer review as longer article types.
Your paper is either too short, or only presents a short, focused result that the journal does not consider a "full" paper. It's hard to know, as they won't exactly lay out the definitions of papers they don't accept, but you may want to look at similar journals to see if there is a field-based consensus for what a sufficiently large finding is.