probably smaller core getting saturated , i will adapt to PNP NPN design
http://pindiagram.blogspot.com/2012/02/dc-to-ac-inverter.html
That design is very inefficient due to the huge losses with the 555 driving emitter followers.
If you use the original design posted at the start of the thread, with a centre tapped transforer two MOSFETs, the core is less likely to be saturated, since it will be alternately magnetised with opposite polarities, thus be used more efficiently.
If you need a smaller solution forget the bulky, inefficient inverter. Use a 12V LED and a small, lightweight, efficient boost converter.
yes the core saturation was the issue
--its resloved! ---

-- used Centertapped transformer and dual mosfets
-- Achieved good brightness at
230mA so 2200mAH pack will last
8-10hour with 2S4P we can get 20hour backup , which is huge
developed an ECO mode , slight less brightness at
155mAon 2200mA ECO mode can pull till 12Long hours
All this with palm sized inverter.
test shown a 500mA transformer can take two 7Wbulbs ( lower brightness )
but this centertap is very good for one 7W