Paperclip attachment size validation fails, but size is valid?

Question

My model

class Article < ActiveRecord::Base
  LOGO_TYPES = ['image/jpeg', 'image/png', 'image/gif']
  VALID_LOGO_MESSAGE = 'Please attach a valid picture file containing your organization logo.  Valid formats are JPG, PNG, and GIF'

  has_attached_file :logo, :content_type => LOGO_TYPES,
  :path => ":rails_root/app/assets/images/uploads/articlelogos/:id/:style/:basename.:extension",
  :url => "/assets/uploads/articlelogos/:id/:style/:basename.:extension"

  validates_attachment_content_type :logo, :content_type => LOGO_TYPES, :message => VALID_LOGO_MESSAGE
  validates_attachment_size :logo, :less_than => 1.megabytes
end

When I try to upload a valid, 6k .gif logo using Paperclip, I get:

Logo_file_size file size must be between 0 and 1024 bytes

Logs as request hits :

Started PUT "/articles/74" for x.x.x.x at 2012-01-15 17:44:20 -0600
  Processing by ArticlesController#update as HTML
  Parameters: {"utf8"=>"✓", "authenticity_token"=>"uI+OXmV3B/crOrUopbBxpRs242PzMVJY1uM/QBnHdlE=", "article"=>{"logo"=>#<ActionDispatch::Http::UploadedFile:0x000000081d70b0 @original_filename="test_logo.gif", @content_type="image/gif", 
  @headers="Content-Disposition: form-data; name=\"article[logo]\"; filename=\"test_logo.gif\"\r\nContent-Type: image/gif\r\n", 
  @tempfile=#<File:/tmp/RackMultipart20120115-30606-1wwkr21>>}, "commit"=>"Upload Now", "id"=>"74"}

And I added some debugging to show the tempfile

params[:article][:logo].inspect: #<File:/tmp/RackMultipart20120115-30606-1wwkr21>>
params[:article][:logo].size: 6531

So it is clearly being stored on the server, but Paperclip is failing to check the size properlly.

This works on my local Mac in development mode, but fails on production under Ubuntu.

Any ideas? Thanks!

Update - removing size validation enables the file to be uploaded and stored as usual