The speaker looks back on youth and lists the things that once fascinated them: ants, clouds, birds. The first three stanzas repeat the same pattern, so the poem feels like a child's habit of naming favorite things one after another. Each item is a piece of the natural world, small and everyday, from the ground to the sky.
The last stanza breaks that pattern. Here the speaker says that nothing was admired more than "the daily lives of people." The grammar is a little unusual, which gives the line a quiet, slightly hesitant tone, and the trailing dots let the thought drift off before "when I was young" returns once more.
The title puts "Small" in quotation marks, which suggests the speaker doesn't really think ordinary lives are small at all. The mood is gentle and nostalgic, with a touch of wonder. Ants, clouds, and birds were fascinating, but people going about their days mattered most.