Some stanzas are longer than others in the poem "The Key of the Kingdom" by Ed Reed because the poet is using these stanzas to show the course - the flow - of life. The stanzas and their length correspond somewhat to the various stages many people go through in life, if they live an average life expectancy.
The stanzas are also of varying length so that the poem is not predictable, because, in general, life is not predictable. Therefore, there is no standardized stanza length ( and look) in this poem.
The first stanza is long and flows with the easy grace of childhood, where we engage the world and learn about and explore the world around us.
As the poem progresses to the end, the last two stanzas are shorter. This signifies that one is approaching the "autumn" or even the "winter" of their days and the youthful spring and summer seasons are gone. Now time is shorter, and of the essence; we feel we must redemm the time, making better use of the dwindling years - which can still be enjoyable and very productive years in many cases.
The poet indicates that we become wiser and more mature at the later stages of life. However, he does end that last short stanza on a somewhat dour note as he says:
"Age is the grave yard
Of all our youthful hopes."