Docker 1.7比Docker 1.5占用更多的映像空间

最初我ubuntu Trusty通过Vagrant安装了docker 1.5 。

然后我创建了新的Ubuntu Trusty，现在docker版本是1.7。

我尝试使用此Dockerfile构建Postgres容器的图像

FROM ubuntu:trusty

RUN echo "deb http://archive.ubuntu.com/ubuntu trusty main universe" > /etc/apt/sources.list

RUN apt-get -y update

RUN apt-get -y install wget openssh-server

RUN apt-get install -y supervisor

RUN mkdir -p /var/run/sshd

RUN wget --quiet --no-check-certificate -O - https://www.postgresql.org/media/keys/ACCC4CF8.asc | apt-key add -

RUN echo "deb http://apt.postgresql.org/pub/repos/apt/ trusty-pgdg main" >> /etc/apt/sources.list

RUN apt-get -y update

RUN apt-get -y upgrade

RUN locale-gen --no-purge en_US.UTF-8

ENV LC_ALL en_US.UTF-8

RUN update-locale LANG=en_US.UTF-8

RUN apt-get -y install postgresql-9.3 postgresql-contrib-9.3 postgresql-9.3-postgis-2.1 postgis

RUN echo "host    all             all             0.0.0.0/0               md5" >> /etc/postgresql/9.3/main/pg_hba.conf

RUN service postgresql start && /bin/su postgres -c "createuser -d -s -r -l docker" && /bin/su postgres -c "psql postgres -c \"ALTER USER docker WITH PASSWORD 'docker'\"" && service postgresql stop

RUN echo "listen_addresses = '*'" >> /etc/postgresql/9.3/main/postgresql.conf

RUN echo "port = 5432" >> /etc/postgresql/9.3/main/postgresql.conf

EXPOSE 5432 22

ADD start.sh /start.sh

RUN chmod 0755 /start.sh

cmd ["supervisord", "-n"]

现在当我用docker 1.5检查我的ubuntu时

Filesystem           Size  Used Avail Use% Mounted on
/dev/sda1             40G  2.3G   36G   6% /
none                 4.0K     0  4.0K   0% /sys/fs/cgroup

当我检查docker 1.7时

Filesystem           Size  Used Avail Use% Mounted on
/dev/sda1             40G   14G   24G  38% /
none                 4.0K     0  4.0K   0% /sys/fs/cgroup

我专门测试了仅一张图像，而服务器上没有其他图像。

我什至检查了磁盘使用情况及其来源 /var/lib/docker/vfs/dir

我想知道1.7多数民众赞成在什么变化导致