In "Remember," what is the effect of compressing such a heavy topic (death and remembrance) into a sonnet?

In Christina Rossetti's sonnet "Remember," the speaker asks the addressee of the poem to remember her after she is dead, unless remembering should be too painful. It is a sad poem about two lovers being separated by death and about the pain consequent of their separation. This is indeed a rather profound topic to explore in just fourteen lines.
One effect of writing about this subject in just fourteen lines is that the language is rather simple and direct. The speaker doesn't digress or indulge in unravelling complex images, but, because she only has fourteen lines, expresses herself simply and directly. For example, the opening line, "Remember me when I am gone away," is at once plain and plaintive, in large part because of its simplicity.
Because the poem is so short, the imagery that Rossetti employs is also simple and suggestive. For example, in the third line, we have the image of the two lovers holding hands, and in the second line we have the metaphorical image of "the silent land," which serves as a euphemism for death. The simplicity of the first image derives from the fact that it is also a universally recognized image that everybody can easily and quickly empathize with. The second image is less immediately accessible, but is still nonetheless a rather simple, suggestive image which immediately connotes the emptiness and desolation of death.
Also, because the poem is only fourteen lines, it is more accessible and thus more widely read than it might otherwise have been. It is likely, for example, that more people have read "Remember" than have read one of Rossetti's other famous, much longer poems, "Goblin Market." Thus, one effect of writing such a short poem is simply that it has become more widely read than it might otherwise have been.