I start with the finished dimensions and then build the art to fit that dimension. It sounds like you didn't do that. Or perhaps you're offering the poster in more than one size? If both sizes are offered, I think the content should be consistent.
Larger posters are for viewing from further away. Adding content is going to lead to a busy design. White Space is king. Clutter is nasty.
The link below shows the best 'Bad Graphic Design' on Pinterest. A good percentage are over filled and cluttered.
Consistency is important but reduce the content as you start reducing the poster size. Use only the content that is important.
"One size does not fit all"