Device-to-Device　(D2D)　communication　with　caching　technology　has　emerged　as　a　promising　technique　for　offloading　the　traffic　and　boosting　the　throughput　of　the　fifth　generation　(5G)　cellular　networks.　The　combined　impact　of　cache　memory　size　of　user　equipments　(UEs)　and　content　sizes,　which　are　two　crucial　factors　in　D2D-assisted　caching　networks,　were　usually　ignored　in　the　existing　researches.　In　this　paper,　an　optimization　algorithm　is　proposed　to　maximize　the　cache　hit　probability　and　cache-aided　throughput,　with　the　consideration　of　various　cache　memory　sizes　of　UEs　and　content　sizes.　Firstly,　users　are　grouped　according　to　the　cache　memory　sizes　and　the　content　sizes.　Then　the　general　mathematical　expressions　for　the　optimization　of　cache　hit　probability　and　cache-aided　throughput　with　the　constraints　of　cache　memory　sizes　and　content　sizes　are　obtained.　Subsequently,　a　Packet　Cache　Strategy　(PCS)　algorithm　is　proposed　to　obtain　the　caching　probability　matrix　with　the　maximum　cache　hit　probability　and　cache-aided　throughout　by　taking　user　caching　probability　of　a　file　as　a　variable.　Finally,　numerical　results　show　that　the　sizes　of　the　requested　files　affect　the　caching　willingness　of　users,　and　the　proposed　PCS　can　achieve　the　highest　cache　hit　probability　and　the　best　cache-aided　throughput　comparing　with　two　other　existing　methods.