Because of their recharge and shape.
The Draconic Bloodline breath weapon is more powerful (70 average damage vs 54) because it can only be used 3 times per day and affects a smaller area. The Form of the Dragon breath weapon can be used every 1d4 rounds, and affects a 50' cone or 100' line, versus the bloodline breath weapon's 30' cone or 60' line.
Note that as far as I can tell nothing stops you from using your Draconic Bloodline breath weapon while you're in Form of the Dragon; it's not that your breath weapon suddenly does less damage, it's that you gain another breath weapon with lower damage but a much faster recharge and larger area.
From the PFRD (emphasis mine):
Form of the Dragon I
You become a medium chromatic or metallic dragon. You gain... a breath weapon. Your breath weapon and resistance depend on the type of dragon. You can only use the breath weapon once per casting of this spell. All breath weapons deal 6d8 points of damage and allow a Reflex save for half damage. In addition, some of the dragon types grant additional abilities, as noted below.
"All breath weapons", in all 3 versions of the spell, means "any breath weapon you got from this spell, regardless of which kind of dragon you chose", rather than "all abilities you have that happen to be called a breath weapon".
Regarding weapons:
From pg. 144 of the Core Rulebook:
Cost: This value is the weapon's cost in gold pieces (gp) or silver pieces (sp). The cost includes miscellaneous gear that goes with the weapon, such as a scabbard or quiver.
This cost is the same for a Small or Medium version of the weapon. A Large version costs twice the listed price.
and a little further along
Weight: This column gives the weight of a Medium version of the weapon. Halve this number for Small weapons and double it for Large weapons.
Masterwork is an additional flat fee of 300gp, regardless of size.
The damage for large weapons is listed on table 6-5, on pg. 145 of the Core Rulebook.
Regarding armour:
Table 6-8, on pg. 153 of the Core Rulebook, covers the cost and weight increases for different sizes. For a large creature, the cost and weight would be double.
Masterwork is an additional flat fee of 150gp, regardless of size.
Regarding adventuring equipment:
There is no rule that covers changes for all the different sizes of the items that are available. One of the reasons for this is that among the core races, and those presented in the Advanced Race Guide, none of them are Large. The rules cover Small and Medium sizes because 99% of PCs will be those sizes.
My recommendation would be to follow the rules for weapons and armour. Those are the more important parts of your equipment and the rules are clear regarding large sizes.
As for the rest of the equipment, don't over-complicate things. If an item has a specific ruling regarding its size, weight, or any other aspect, then follow that ruling. Otherwise, simply follow the standard information that is listed for the item. And remember, there is nothing that requires you to have to buy the larger versions of items. I see no reason why you would struggle with a normal sized pouch.
Finally, have a quick chat with your DM about this to clear up any questions you may have and also to make sure that you are both on the same page.
Best Answer
Unfortunately, no explicit rules for the purchasing of normal scabbards, lead lined or otherwise, are given, at least in the SRD. Nonetheless, we can get a fairly precise answer.
An ounce of lead is a costless focus component for the spell Lead Plating found in Kobolds of Golarion. This means that lead is effectively free, at least in the quantities you need to line a scabbard. That just leaves the cost of labor if you aren't willing to do the lining yourself. At this point we have to determine whether a lead-lined scabbard requires a scabbard and lead as raw materials (in which case it costs twice as much as a scabbard does) or whether it requires a scabbard's raw materials (i.e. half a scabbard, in which case the cost is no different than a regular scabbard) to complete.
Assuming you use a normal scabbard, things are still complicated. Scabbards appear to be either free, or possibly included as part of your clothing/weapon/something else, which puts the cost into GM discretion up to twice the cost of the clothes or weapon the scabbard came with.
If your GM is charging you some incredibly unreasonable price for a regular lead-lined scabbard, buy a lead-lined combat scabbard instead, which costs a maximum of 2gp (assuming it's not sharp. Sharp ones go up to 20 gp in the worst case scenario).
If you happen to be playing with the 3rd party Pure Steam Campaign Setting, there are much more explicit rules for lead lining stuff and it is, indeed, free.
If you are the GM, the price should be no different than a normal scabbard, and thus free if you don't track equipment very closely and less than a silver piece but more than a copper piece if you do.