https://github.com/thoughtbot/paperclip/issues/1085 this is cheap hack to get the convert_options to happen before transformation so the image size stays as defined even if there are convert options

paperclip_convert_options_before_transformation.rb

raise "Expecting Paperclip version 3.3.1 since we are monkey patching it in application.rb" unless Paperclip::VERSION == "3.3.1"

module Paperclip
  class Thumbnail < Processor
    def make
      src = @file
      dst = Tempfile.new([@basename, @format ? ".#{@format}" : ''])
      dst.binmode

      begin
        parameters = []
        parameters << source_file_options
        parameters << ":source"
        if transformation_command.any?{|cmd|cmd.match(/resize/)}
          parameters << convert_options
          parameters << transformation_command
        else  
          parameters << transformation_command
          parameters << convert_options
        end
        parameters << ":dest"

        parameters = parameters.flatten.compact.join(" ").strip.squeeze(" ")

        success = convert(parameters, :source => "#{File.expand_path(src.path)}#{'[0]' unless animated?}", :dest => File.expand_path(dst.path))
      rescue Cocaine::ExitStatusError => e
        raise Paperclip::Error, "There was an error processing the thumbnail for #{@basename}" if @whiny
      rescue Cocaine::CommandNotFoundError => e
        raise Paperclip::Errors::CommandNotFoundError.new("Could not run the `convert` command. Please install ImageMagick.")
      end

      dst
    end

  end
end

@jotto Thanks for your help with this. I offer back to you the following enhancements. I'm using this with Spree, so currently stuck to Paperclip 2.8.0, so changes are reflected.

I also have need to add some flags after the -resize flag, so I have included some changes in transformation_command to further hack your monkey patch. Thanks again.

raise "Expecting Paperclip version 3.3.1 since we are monkey patching it in application.rb" unless Paperclip::VERSION == "2.8.0"

module Paperclip 
  class Thumbnail < Processor

    # Returns the command ImageMagick's +convert+ needs to transform the image
    # into the thumbnail.
    def transformation_command
      scale, crop = @current_geometry.transformation_to(@target_geometry, crop?)
      trans = []
      trans << "-coalesce" if animated?
      trans << "-resize" << %["#{scale}"] unless scale.nil? || scale.empty?
      trans << "-crop" << %["#{crop}"] << "+repage" if crop
      trans << '-layers "optimize"' if animated?
      trans << "-extent" << %["#{scale}"] unless scale.nil? || scale.empty?
      trans << '-unsharp 0x0.5 -enhance'
      trans
    end

    def make
      src = @file
      dst = Tempfile.new([@basename, @format ? ".#{@format}" : ''])
      dst.binmode

      begin
        parameters = []
        parameters << source_file_options
        parameters << ":source"
        if transformation_command.any?{|cmd|cmd.match(/resize/)}
          parameters << convert_options
          parameters << transformation_command
        else  
          parameters << transformation_command
          parameters << convert_options
        end
        parameters << ":dest"

        parameters = parameters.flatten.compact.join(" ").strip.squeeze(" ")

        success = Paperclip.run("convert", parameters, :source => "#{File.expand_path(src.path)}#{'[0]' unless animated?}", :dest => File.expand_path(dst.path))
      rescue Cocaine::ExitStatusError => e
        raise Paperclip::Error, "There was an error processing the thumbnail for #{@basename}" if @whiny
      rescue Cocaine::CommandNotFoundError => e
        raise Paperclip::Errors::CommandNotFoundError.new("Could not run the `convert` command. Please install ImageMagick.")
      end

      dst
    end
  end
end