// --- Simply Backpacks ---
//Uncommon
recipes.removeByRecipeName("simplybackpacks:uncommon");
recipes.addShaped("simplybackpacks_uncommon", <simplybackpacks:uncommonbackpack>, [[<minecraft:gold_ingot>, null ,<minecraft:gold_ingot>], [<ore:dyeYellow>, <simplybackpacks:commonbackpack>.marked('commonpack'), <ore:dyeYellow>], [<ore:chest>, <minecraft:gold_block>, <ore:chest>]], function(out, ins, cinfo) {
var inTag = ins.commonpack.tag;
print("Backpack Current Size: " + inTag.inv.Size.asString());
val outTag = {
inv:
    {
        Size : 33,
        Items : inTag.inv.Items
    },
filter:
    {
        Size : 16,
        Items : inTag.filter.Items
    }
} as IData;
//var test = (inTag.inv + sizechange) as IData;
//print("Backpack Size change to: " + test.asString());
//var test2 = inTag + test as IData;
print("Finalize Change to: " + outTag.asString());
return out.withTag(outTag);
}, null);

crafttweaker.log
-------------------------------------
[SERVER_STARTED][CLIENT][INFO] Backpack Current Size: 18
[SERVER_STARTED][CLIENT][INFO] Finalize Change to: {inv: {Size: 33, Items: [{Slot: 0, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 1, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 2, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}]}, filter: {Size: 16, Items: []}}
[SERVER_STARTED][SERVER][INFO] Backpack Current Size: 18
[SERVER_STARTED][SERVER][INFO] Finalize Change to: {inv: {Size: 33, Items: [{Slot: 0, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 1, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 2, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}]}, filter: {Size: 16, Items: []}}
[SERVER_STARTED][CLIENT][INFO] Backpack Current Size: 18
[SERVER_STARTED][CLIENT][INFO] Finalize Change to: {inv: {Size: 33, Items: [{Slot: 0, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 1, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}, {Slot: 2, id: "minecraft:diamond", Count: 1 as byte, Damage: 0 as short}]}, filter: {Size: 16, Items: []}}